October 3, 2024
The sheer number and severity of recent extreme weather events around the world is a sign of things to come. How do these come about?
Are we going through an exceptional period of extremes, or is this the new normal?
This post is rather longer than usual, so I shall begin with a list of contents. I hope readers find it interesting and enlightening.
A look at the world’s recent weather trends
First, a brief recap from last week’s post “Earths Energy Imbalance”.
Global warming comes from an excess of energy, with current magnitude 484 TW and increasing. The Earths energy budget is far from being in balance, with some increasingly dangerous consequences for us humans.
There are a number of contributors and detractors to this excess energy:
- greenhouse gases – until we either reduce the concentrations through (i) climate intervention actions / geoengineering; or (ii) CO2 capture plants – both of which are likely decades away.
- water vapour – ironically the main “side benefit” of warmer air is that it carries more water vapour, which in the round has a cooling effect. Although it also brings more latent energy and therefore more extreme weather.
- aerosols – aerosols in the atmosphere are formed from particulates – dust, soot (black carbon), organic carbon, sand, sea salt and many others. These act as seeding agents to form clouds, increasing albedo (reflection of solar radiation back to space) and partially offsetting the ice-albedo effect.
This colossal amount of energy has to go somewhere, resulting in temperature rises – in the oceans, land, atmosphere. Subject to normal climate variability, it is this extra heat (currently ~ 0.95 W per m2), colossal in energy terms, that acts as an additional excitor or driver of stronger atmospheric and oceanic circulations.
In terms of weather effects, increased temperatures contribute to a greater disparity in wet areas and dry areas – in the main, wet areas are getting wetter, dry areas are getting drier. https://news.climate.columbia.edu/2017/05/31/in-a-warmer-world-expect-the-wet-to-get-wetter-and-the-dry-drier/
Naturally, the interactions are more complex in practice, as explored further in Appendix 1 (Atmospheric Rivers) and Appendix 2 (ENSO and atmospheric circulations).
Land also plays a key role in the exchange of energy, water and aerosols between the land surface and atmosphere. The terrestrial biosphere also absorbs almost 30% of anthropogenic CO2 emissions.
But these vital functions are under attack, from increasing heat stress, increased droughts and wildfires, floods, deforestation and other environmental and human pressures. As well as feedback effects driven by the above.
Climate and Extreme Weather
“Climate change is already affecting every region on Earth, in multiple ways. The changes we experience will increase with additional warming”
Panmao Zhai, IPCC Working Group Co-Chair
Climate can be thought of as the average weather conditions we experience, or the statistics of weather over long periods of time. Climate varies naturally on a whole range of timescales and together with the Earth’s energy imbalance, these variations can have more profound impacts on the Earth. Because of its importance, climate has been intensively studied over the past 20–30 years, leading to increased knowledge of ocean–atmosphere interactions, especially over the tropics.
There are many factors that can contribute to extreme weather, including:
- Natural variability in the climate
- Temperature increases in the ocean, on land and in the atmosphere
- And more generally, anthropogenic climate change, as manifested by faster warming, longer warming seasons, heat waves, intensification of the water cycle (more erratic rainfall, more droughts), ocean acidification and reduced oxygen levels and so on.
- Land–ocean heat contrast – changes in which can lead to heavy precipitation in e.g. East Asian coastal regions
- Atmospheric evaporative demand (or Vapour pressure deficit, VPD) – higher demand can dry out soils and evaporate greater quantities of water over land or sea
- The spatial distribution of temperatures – which affects ocean and air pressures and pressure gradients, and therefore atmospheric and ocean circulations.
- Changes in atmospheric circulations, such as:
o “Hadley circulation” – the Hadley circulation is a global-scale North-South tropical atmospheric circulation which affects where tropical and other cyclones occur. Air rising near the equator flows poleward near the tropopause, cooling and then descending in the subtropics (around 25 degrees latitude), before returning equatorward near the surface.
o “Walker circulation” – The Walker Circulation is the East-West counterpart to the Hadley circulation. It regulates global exchange of momentum, heat and water vapor within the tropics via massive overturning motions. It is a thermal circulation that relies on temperature differences to drive the rising and sinking branches. It plays an important role in the balance of equatorial atmospheric energy, and in determining the characteristics of tropical weather.
- Changes in ocean-atmosphere interactions, such as:
o The El Niño–Southern Oscillation (ENSO) – an ocean-atmosphere interaction. The ENSO can affect the Walker circulation (and Hadley circulation) over the entire tropics, impacting rainfall near the equator across multiple continents from Africa to Asia to South America. During ENSO events, the Walker circulation gets pushed or pulled around (El Niño) or sent into overdrive (La Niña) across the Pacific.
o Atmospheric Rivers” (AR) – another form of ocean-atmosphere interaction. ARs are massive storms – fast-flowing currents of air that form in warm ocean waters as seawater evaporates. ARs carry huge bands or columns of condensed water vapour in the lower part of the atmosphere, which moves from the tropics to the cooler latitudes, where it comes down as rain or snow. The downpours from ARs can come with enough force and duration to cause significant flooding, snowfalls and landslides.
World Weather
Water Cycle Extremes – Droughts and Pluvials
The above video from NASA shows the recorded weather extremes over the period 2002-22, based on satellite observations. Dry events are shown as red spheres and wet events as blue spheres, with earlier years being shown as lighter shades and later years as darker shades. The volume of the sphere is proportional to the intensity of the event, a quantity measured in cubic kilometer months. The plots at the bottom of the figure show that the total intensity of extreme events increased as global temperatures increased.
The large blue dots represent floods and rainfall events, with red dots for droughts.
As water vapour levels increase in some areas, and fall in others, this leads to
- more frequent and widespread flooding, repeat flooding, stronger cyclones and greater rainfall; and
- the opposite effect in other areas – more frequent and widespread heatwaves and droughts, dry periods and wildfires.
Whilst floods are not uncommon during monsoons / rainy seasons, what we can see is that the rains have become more erratic – with massive rainfall in a short space of time, followed by long periods of dryness.
What is driving this phenomenon?
Increases in atmospheric water vapour amplify the global water cycle. They contribute to making wet regions wetter and dry regions drier. The more water vapour that air contains, the more energy it holds. This energy fuels intense storms, particularly over land, resulting in more extreme weather events.
But more evaporation from the land also dries soils out. When water from intense storms falls on hard, dry ground, it runs off into rivers and streams, instead of dampening soils. This increases the risk of drought.
Where does the additional atmospheric water vapour come from?
- on land, water is disappearing skyward through increased evapotranspiration* – higher temperatures imply higher evapotranspiration.
- along the coasts and at sea, seawater is evaporating from the oceans – warmer oceans imply greater seawater evaporation.
- the VPD (vapour pressure deficit) of the atmosphere is also relevant – higher temperatures increase the VPD, increasing the capacity of the atmosphere to absorb more water vapour. This is estimated at about 7% additional water vapour for each 1°C of ocean warming.
* Evapotranspiration is the total water loss to the atmosphere from a land surface, including (i) the water vapor evaporating from the soil surface and plant surfaces, together with (ii) that transpired from within plant surfaces.
* VPD is the difference between the amount of water vapour that the atmosphere can retain (which rises with temperature) and the amount of water vapour it does contain (relative humidity).
Uncertainty in the future evolution of tropical rainfall is linked to our understanding of how this water vapour is being transported from the tropics and across continents. Recent advances are resolving this uncertainty, as we model and better understand the processes involved.
We look more closely at the related processes in Appendix 1 (Atmospheric Rivers) and Appendix 2 (ENSO and atmospheric circulations).
Extreme Weather in the last month
In the last month, and for many the last week, we have observed extreme weather events all over the world, with millions displaced from their homes.
A non-exhaustive list of extreme weather examples follows:
ASIA
EUROPE
AFRICA
- Floods across 14 central and West African nations killed about 1,000 people and made 4 million homeless, including Chad, Nigeria, Cameroon, Mali and Niger:
- in Niger, over 841,000 people impacted and over 400,000 displaced
- in Mali the worst floods since the 1960s
- in Nigeria, widespread devastation, with the overflowing of the Alau Dam leading to the worst floods in 30 years.
- in Cameroon
- and Chad, more than 160,000 homes and 259,000 hectares of farmland destroyed, as the Chari River swells 35% above normal. By early October, levels could reach 8.6 m, surpassing the devastating 2022 floods.
- Flooding in Morocco and Algeria
THE AMERICAS
- torrential rains and flooding in Southern US states – Florida, Georgia, North Carolina, South Carolina, Tennessee, Kentucky. With Hurricane Helene passing over water 2 C warmer than normal, it went from a Cat 1 to a Cat 4 hurricane by landfall, with 15-20 foot storm surges, 3 million homes without power, and damages from floods, landslides and winds expected at $100 billion. Typically a low rainfall venue, Asheville in North Carolina received nearly 4 feet of rain, with mudslides drowning the areas below and more than 400 roads closed. Atlanta has had its wettest 3 day period in 104 years.
- droughts, heatwaves and wildfires throughout South America, the second largest wildfires this century
- Hurricane John crashed into Guerrero state, Mexico, dropping nearly 3.4 feet of rain with devastating floods in many places.
Extreme Weather in the last year
And in the last year we have observed major floodings (which tend to grab the headlines) but also heatwaves and extreme droughts, across the world.
A non-exhaustive list of extreme weather examples follows:
ASIA
– in Pakistan since late Feb 2024, in a country still recovering from the 2022 floods which submerged 1/3 of the country, affecting 33 million people
– in Afghanistan in May 2024
– in West Sumatra, Indonesia in May 2024
– in Bangladesh in Aug 2024
MIDDLE EAST
EUROPE
AFRICA
- in Sudan
- in DRC, the worst floods in 60 years
- in Kenya and Tanzania
- according to African Risk Capacity Group, 29 African countries witnessed disasters in 2023.
- extreme heat in Nigeria and Ghana
- in Zimbabwe, a state of disaster was declared, due largely to El Nino-induced drought.
- in southern Africa, extreme drought affected Zambia and Malawi, with both countries declaring national disasters, with Botswana, Angola, Mozambique and Madagascar also affected.
- in South Africa, severe floods in KwaZulu-Natal and the Eastern Cape
THE AMERICAS
- increased frequency and strength of hurricanes making landfall in the USA – Hurricane Helene’s landfall gives the USA a record 8 major Atlantic hurricane (Cat 4 or Cat 5) landfalls in the past 8 years. The previous 8 before that occurred over 57 years. The increased strength comes from warmer surface water and greater water vapour feeding the hurricanes.
- record temperatures and heatwaves, with some extreme records e.g. 47.2 C in Phoenix, Arizona, 5 C above the previous record 1992 high of 42.2 C.
- in Brazil, a total of 2.3 million people affected by the torrential rain and floods of May 2024.
- in Jamaica, Haiti and the Dominican Republic, torrential rainfall struck in Nov 2023, with at least 21 deaths in the latter, and a new record rainfall of 431.0 mm in one day.
- in Uruguay (Mar 2024) repeated flash flooding in Montevideo and the Canelones, Florida, Río Negro, San José and Soriano areas.
- in Argentina (Mar 2024)
Climate Attribution
A 2021 Carbon Brief study mapped over 350 peer-reviewed studies of weather extremes, finding that extreme events have increased in the last 10 to 15 years, with climate change attributed as making the majority of these events more likely and more severe:
- 70% of 405 extreme weather events were made more likely or more intense by climate change.
- 92% of 122 attribution studies of extreme heat found that climate change made them more likely or more severe.
- 58% of 81 rainfall studies found that human activity made them more probable or intense, and
- 65% of 69 drought events were also exacerbated by climate change.
More environmental impacts – sea level rise, melting permafrost or snowpack, extreme heat, ocean acidification – can increasingly be attributed to climate change and sources of emissions.
“When the science changes, when a body of knowledge to which a responsible professional is expected to keep up with and understand and pay attention to—when that changes, it changes what they have to do to protect people. It changes the standard of care.”
Lindene Patton, Partner at Earth & Water Law
In this respect, the Climate Attribution Database has been established by the Sabin Center for Climate Change Law and Lamont-Doherty Earth Observatory.
The emerging sub-field of climate attribution science can be used to defend climate regulations that are challenged as being too stringent, or to establish standing to sue. It can help hold emitters liable and sue governments for not sufficiently regulating GHG emissions.
And it also can provide some new insights into the impacts of climate change, as a tool to help in risk mitigation, and to educate and motivate communities and stakeholders to take action.
For example: NOAA is attempting to mitigate risks through its Coastal Flood Exposure Manager and its storm surge mapping tool – identifying which areas are most impacted.
Similarly, FEMA offers a flood risk map service (searchable by zip code) together with flood zone FAQs.
The New Normal
Is this the new normal, or are we going through an exceptional period of extremes?
“Maybe 2024 is the best year of the ones that are coming, as incredible as it may seem …. The climate models show a big share of the [Amazon] biome is going to become drier.”
Erika Berenguer, senior research associate, University of Oxford.
Yes the current period has been exceptional, but this is the new normal unfortunately.
As with the Earths Energy Imbalance, there is a excess energy in the system. As global temperatures continue to rise, that excess energy must be stored somewhere:
- as increased latent energy in e.g. water and water vapour
- as increased kinetic energy in e.g. winds and ocean currents
- as increased heat throughout the planet’s layers
- as increased potential energy e.g. pressure gradients, the build-up of electrical charges in the atmosphere, to be later discharged as lightning.
All of these factors will see new records in future for land and ocean temperatures, the take up and transport of water vapour, prolonged droughts – a continuation of current trends.
A New Reality – Watching the climate crisis play out in real time
Of course many of these events have occurred in flood-prone regions, or could be categorised as within natural variability, or human-caused, or due to unusual local weather effects. However, the sheer number and synchronicity of them tells us something more.
“While the extreme events we face are hard to predict with precision, we do know we will see more of them”
Jeffrey Schlegelmilch, National Center for Disaster Preparedness, Columbia Climate School
As global heating continues, there is a deteriorating overall picture and a trend of increasing frequency and severity. A new reality where, on average, these effects are now more widespread, more frequent and more severe than in the past. With new records being set in respect of, inter alia, temperature, rainfall, drought, number of wildfires, environmental and economic damage.
In short, events with probabilities of 1 in 500 or 1000 are becoming much more widespread, with their probabilities of occurrence being made 10-50 times more likely by the effects of anthropogenic climate change.
- As the ocean warms, tropical cyclones like Hurricane Helene or Typhoon Yagi have the same frequency BUT the dangers are increasing
- Lesser cyclones are also becoming more dangerous, so there are more risk events in future
- They are moving more slowly
- They are growing larger and stronger, fuelled by the additional water vapour
- They are intensifying more quickly
- They are dumping a lot more rain, with more severe impacts
- Their storm surges are greater
The Policy Imperative:
“The best time to plant a tree is twenty years ago. The second best time is now.”
Attributed to an old Chinese proverb
Yes the time for action was indeed twenty years ago, but action today will still mitigate the worst of what is around the corner.
- Governments no longer have the option of delaying climate action – they are either part of the solution or part of the problem
- They are going to have to make some tough choices about fossil fuels (and doing nothing is also a choice)
- Most large producers continue to go a middle way, both:
o expanding their oil and gas footprint – at the margin, or more centrally
o expanding renewable energies at speed
For example the UAE, Saudi Arabia, Brazil, USA. This is often seen as the reality of dealing with Big Oil, striking bargains in order to make strong progress on clean energy.
Meanwhile, the world will continue to bear the increasing environmental impacts of colossal fossil fuel production, and the implied increasing amounts of pollution, CO2 and methane.
One positive is that our ability to monitor and understand the climate system and the underlying physics, has improved considerably in recent years. This gives us a clearer perception of what is happening, and with that, more ability to recognise warning signs and take actions.
Conclusion
We can now see the path that Nature is taking more clearly – in terms of major events, their frequency, severity, human and environmental impacts, and the costs of restoration and adaptation. Nature is following the laws of physics, and the effects of those are beginning to ramp up.
Unlike the extreme weather events which do seem to be “everywhere, all at once”, there is some remarkable progress being made – from the mass uptake of electric vehicles, to renewable energies going mainstream (c.5% of global GDP in the next year), to climate legal precedents being set, to more and more countries and companies setting Net Zero policies. It will be incremental but accelerating progress. And with it a greater understanding among all of environmental impacts in our everyday lives.
Time is against us, but this is the world we live in. A global circular economy won’t be built in a day – it is gradual process and might take 10-15 years. Similarly the move to a clean energy system might take 20-30 years, depending on technological advances and the future path of climate negotiations, amid the new realities of extreme global weather events and their increasing effects on food security and welfare.
“Without actions that address the root problem of humanity taking more from Earth than it can safely give, we’re on our way to the potential collapse of natural and socioeconomic systems and a world with unbearable heat and shortages of food and freshwater.”
“By 2100, as many as 3 billion to 6 billion people may find themselves outside Earth’s livable regions, meaning they will be encountering severe heat, limited food availability and elevated mortality rates.”
Dr Christopher Wolf, Oregon State University
In the short-term we must take direct remedial action on the ground, with direct climate finance for major restoration projects in the time remaining. Policymakers and financiers need to have a major re-think of what and how they are funding. The consequences of the current path are now an inescapable reality. And they need to do this now.
Appendices
Appendix 1 takes a closer look at how these extreme weather events are becoming stronger and in some places more frequent, through the mechanism of “Atmospheric Rivers” or Low-Lying Jets (LLJs).
Appendix 2 takes a closer look at the effect of the ENSO (El Niño Southern Oscillation) mechanism and its interaction with the Walker and Hadley circulations, and how this is changing weather in the tropics and further away.
Appendix 3 explores climate change in South America, as a case study. South America has been one of the global hotspots for recent climate extremes. In the past 5 years South America has experienced a 3-year ‘triple La Niña’, followed by a strong El Niño, with an imminent return to the La Niña phase. These phenomena have severely affected the increasingly fragile Amazon region and the Pantanal wetlands, bringing renewed focus and calls to action for this important climate ‘tipping element’.
Recent posts:
COP29 and New Climate Finance Initiatives 2.10.2024
With just 6 weeks to go, the world’s nations are set to decide on a new climate-finance goal, to go beyond the $100 billion per year target set at COP15 in 2009.
Earth’s Energy Imbalance and Global Warming Solutions 25.09.2024
Alongside the massive expansion (and funding) of land restoration and regenerative agriculture schemes and the reduction of CO2, are other solutions on the horizon?
Restoring Natural Capital, Diversity and Resilience 21.09.2024
Despite a slew of international accords over the past three decades, along with 28 COPs, the rate of decline continues. Focus has diverted away from direct physical solutions, towards technological and market solutions supporting short-term decarbonisation. Yet we have the solutions and available funding.
The Environmental Imperative 15.9.2024
The tipping risk elements of the Earth system, their nature and interdependence, and how to mitigate these risks going forwards
APPENDICES
Appendix 1
The Mechanism of “Atmospheric Rivers”
“Atmospheric Rivers” (AR) are another form of ocean-atmosphere interaction. Contributing to many floods worldwide, they carry with them amounts of moisture that are simply colossal.
ARs are massive storms – fast-flowing currents of air that are carrying huge bands of condensed water vapour – which form in warm ocean waters as seawater evaporates. The water vapour forms a band or column in the lower part of the atmosphere, which moves from the tropics to the cooler latitudes, where it comes down as rain or snow. The downpour can come with enough force and duration to cause significant flooding, snowfalls and landslides.
In aggregate, ARs carry around 90% of the total water vapour that moves across the Earth’s mid-latitudes. On average, they have about twice the regular flow of the Amazon, the world’s largest river by water discharge.
In terms of size, (Pacific) ARs might reach as large as 1,500 km long and 500 km wide, transporting huge volumes of water. They may be invisible to the eye, but can be seen with infrared and microwave frequencies, making satellite observations the most useful for observing water vapour and ARs.
Atmospheric pressure gradients and moisture availability allow ARs to persist for periods as long as a week. The magnitude of ARs increases with their duration, related to the size of the moisture uptake region over oceans. For example, long-duration ARs that affect the Western USA have a moisture uptake region that includes most of the northern Pacific.
Other weather systems, such as westerly disturbances, monsoons and cyclones may also carry water vapour and cause flooding.
Cyclones and ARs
Atmospheric rivers may collide with cyclones. When a cyclone interacts with one or more atmospheric rivers, it may pull them along and further strengthen them. One such fast-forming “bomb cyclone” helped spur on the atmospheric rivers that drenched California (see above picture). In January 2023 series of 9 back-to-back atmospheric rivers hit the western coasts of the USA and Canada. Over this period, some 121 billion metric tons of water fell on California alone.
Given the risks of catastrophic floods and landslides, ARs have been categorised into five types based on their size and strength – as with hurricanes. The AR scale is categorized from Category 1 (Cat1) to Category 5 (Cat5).
ARs around the world
Increased frequency and intensity of ARs have been observed in many areas of the world and are expected to increase further in a warmer world.
Increased temperatures and water vapour will make for longer, wider and more intense ARs. Studies have shown that atmospheric water vapour has increased by up to 20% since the 1960s.
ARs are influencing the African monsoon, the Indian monsoon, and the Americas, with massive amounts of rainfall dumped in the Southern US states and sub-tropical South America, both from Pacific and Atlantic ocean sources.
Low-Level Jets and Water Vapor Transport – Africa
In Africa, a series of low-level jets (LLJs) transport the majority of water vapor to central Africa from the Indian Ocean. Flowing from east to west, these are nocturnal LLJs that form in the East African rift system valleys. Flowing through the atmosphere about 500 m above the land surface, the LLJs transport about five times the amount of water as that which flows in the Congo River system.
There is a robust connection between strengthened LLJs and drought in eastern and southern Africa. (Munday et al)
The annual cycle in LLJ-related water vapor transport is in phase with the annual cycle in Congo Basin precipitation. Congo Basin dry seasons occur when LLJ-related water vapor transport is relatively weak, and Congo Basin wet seasons correspond to when LLJ-related transport is strong.
Water vapor transport into Africa is dominated by LLJs, which are the key contributors to the average moisture export and to drought in Eastern Africa. Stronger LLJs are associated with rainfall deficits, both upstream and in the region of the jet. The drier conditions are associated with faster easterly winds.
Low-level westerlies (which occur below 900 hPa pressure) across the Atlantic Ocean coast are also considered to be an important water vapor source for Congo Basin rainfall.
While these LLJs play a key role in determining the aridity of a region, Munday explains that many global climate models aren’t yet able to capture the nuances of these atmospheric movements “The failure of coarse resolution models to capture LLJs is linked with biases in rainfall climatology and variability across the continent”.
The next step in climate model development is to capture the detailed structures of East Africa’s mountains and valleys with a high enough resolution, so that they are able to reflect regional climate both for upcoming seasons and for long-term climate predictions.
ARs and Water Vapor Transport – South Asia
The south Asian monsoon system has large scale flows and strong seasonality in precipitation. The system is projected to transport more moisture under a warming climate, with an increased frequency of ARs making landfall over India. The ARs predominantly occur in the summer monsoon season in India, passing through the Indian sub-continent from west to east.
Warmer sea surface temperatures (SSTs) over the south-central Indian Ocean play a crucial role in the development of ARs. With global warming, evaporation from the Indian Ocean has significantly increased in recent decades. This is due to the increase in VPD (Vapour pressure deficit) – the difference between the amount of water vapour that the atmosphere can retain (which rises with temperature) and the amount of water vapour it does contain (relative humidity). This is estimated at about 7% additional water vapour for every 1°C of ocean warming. https://science.nasa.gov/earth/climate-change/steamy-relationships-how-atmospheric-water-vapor-amplifies-earths-greenhouse-effect/
Of the ten most severe floods during 1985–2020, seven were associated with ARs (1988, 1993, 2004, 2006, 2007, 2013 and 2018). Other major flood events in the summer monsoon season associated with landfalling ARs occurred in 1985, 2000 and 2008. Overall, 70% of India’s major flood events in the summer monsoon season were directly associated with ARs over the 1985–2020 period.
The most devastating flood (June 2013) in Uttarakhand came with over 350 mm (375% departure from the long-term mean) of rainfall with a 3-day accumulated rainfall of more than 400 mm over the region. This event triggered landslides and flash floods. The strong supply of moisture from the Indo-Gangetic (Indus-Ganges) branch of the AR was one of the main drivers of the extreme precipitation.
A total of 574 ARs occurred in the summer monsoon season during 1951–2020, with the frequency of ARs increasing over time. In the last two decades, nearly 80% of the most severe ARs (top 1/3rd AR events) caused floods in India.
ARs should be an integral part of the existing flood early warning systems in India, which can help in adaptation and mitigation.
In South Asia, scientists have associated ARs with up to 56% of extreme precipitation (rainfall and snowfall, although there are limited studies on the region.
In East Asia, there have been more detailed studies on the links between atmospheric rivers and monsoon-related heavy rains. A 2021 study (Park et al) found that up to 80% of heavy rainfall events in eastern China, Korea and western Japan during early monsoon season (March and April) are associated with atmospheric rivers.
“In East Asia there has been a significant increase in frequency of atmospheric rivers since 1940”
“We found that they have become more intense over Madagascar, Australia and Japan ever since.”
Sara M Vallejo-Bernal, Potsdam Institute for Climate Impact Research.
Cat5 ARs are most frequently found in the East Asian summer, along the northwestern boundary of the western North Pacific subtropical high. These Cat5 ARs are found in eastern China, Korea and western Japan, due to the slowly-varying monsoonal flow during the East Asian summer monsoon, which transports a large amount of moisture to the region.
In the Middle East, Iraq, Iran, Kuwait and Jordan were all hit by catastrophic flooding in April 2023, after intense thunder, hailstorms and exceptional rainfall. The skies across the region were carrying a record amount of moisture, surpassing a similar event in 2005.
In South America, Chile was hit by 500mm of rain in just 3 days in June 2023. The sky dumped so much water that it also melted snow on some parts of the Andes mountain, unleashing floods that destroyed roads, bridges and water supplies.
In Australia, parts were hit by what politicians called a “rain-bomb”, with more than 20 people killed and thousands evacuated.
Appendix 2
ENSO and atmospheric circulations.
ENSO (El Nino Southern Oscillation)
The El Niño–Southern Oscillation (ENSO) is a climate phenomenon that fluctuates between three phases: ENSO-Neutral, La Niña or El Niño. La Niña and El Niño are opposite phases
The ENSO phenomenon is an ocean-atmosphere interaction.
The ENSO interacts with the Walker* (east-west) and Hadley** (north-south) atmospheric circulations, driven by the pressure gradient force that results from high pressure over the Eastern Pacific, and low pressure over Indonesia. The ENSO can affect the Walker circulation (and Hadley circulation) over the entire tropics, impacting rainfall near the equator across multiple continents from Africa to Asia to South America. During ENSO events like El Niño and La Niña, the Walker circulation gets pushed or pulled around (El Niño) or sent into overdrive (La Niña) across the Pacific.
**The Hadley circulation is a global-scale North-South tropical atmospheric circulation that features air rising near the equator, flowing poleward near the tropopause, cooling and descending in the subtropics at around 25 degrees latitude, and then returning equatorward near the surface. It affects where tropical and extratropical cyclones occur.
*The Walker Circulation is the East-West counterpart to Hadley circulation. It regulates global exchange of momentum, heat and water vapor within the tropics via massive overturning motions. It is a direct thermal circulation that relies on temperature differences to drive the rising and sinking branches. It plays an important role in the balance of equatorial atmospheric energy and in determining the characteristics of weather and climate in the tropics.
La Niña (warmer water shifted east)
La Niña is associated with an especially strong Walker circulation. It is the cold oceanic and positive atmospheric phase of the ENSO phenomenon, as well as the opposite of El Niño. Under a La Niña, strong winds blow warmer surface water at the ocean’s surface away from South America, across the Pacific towards Indonesia. As this warm water moves further west, cold water from the deep sea rises to the surface near South America.
A stronger sea surface temperature (SST) gradient produces stronger winds across the equatorial Pacific. A weaker gradient results in weaker winds. This can also work in reverse: stronger winds can lead to stronger SST gradients and weaker winds to weaker SST gradients.
El Niño (warmer water in the Central Pacific)
El Niño conditions are established when the Walker circulation weakens or reverses and the Hadley circulation strengthens in the central Pacific. This leads to the development of a band of warmer ocean water in the central and east-central equatorial Pacific, including the area off the west coast of South America, as upwelling of cold water occurs less or not at all offshore.
This warming causes a shift in the atmospheric circulation, leading to higher air pressure in the western Pacific and lower in the eastern Pacific. As a consequence, rainfall reduces over Indonesia, India and northern Australia, while rainfall and tropical cyclone formation increases over the tropical Pacific Ocean. The low-level surface trade winds, which normally blow from east to west along the equator, either weaken or start blowing west to east.
Due to the heating redistribution caused by both La Niña and El Niño, global atmospheric circulation is altered. The movement of so much heat across a quarter of the planet, and particularly in the form of temperature at the ocean surface, can have a significant effect on weather across the entire planet.
Future Intensification of ENSO
Recent Studies show agreement on the future intensification of ENSO’s atmospheric impacts, although there is some debate as to whether ENSO-driven sea surface temperature (SST) variability will also increase in the future. Cai et al. (2022) demonstrate that 34 of 43 models show an increase in ENSO SST variability.
There is also some evidence for an increase in ENSO variability in the observational record, with paleoclimate data suggesting that ENSO variability strengthened post-1950. Coral records also indicate that ENSO variability has been 25 % larger over the last 5 decades compared to the pre-industrial era.
The ENSO can have far reaching impacts – from Indonesia, to the Americas, to Africa – especially when combined with the effects of anthropogenic climate change.
As temperatures rise, the Walker circulation can be expected to be more forceful, and both El Niño and La Niña phases will be stronger on average, and (according to the 0.5 C threshold definition) will occur more frequently.
South America has experienced severe impacts from this confluence of hot, dry and high fire risk conditions. The conditions have been more pronounced in some regions, with widespread heatwaves and droughts. Such conditions often arise from atmospheric blockings – increasing the risk of wildfires.
While El Niño enhances the fire risk in the northern Amazon, dry extremes in other regions appear to be more responsive to La Niña.
The 2020–2023 “Triple La Niña” event
This “triple-dip” La Niña was unusual in that it featured three consecutive years of La Niña conditions, in contrast to the typical 9–12 month cycles of the ENSO. The magnitude of the anomalous sea surface temperatures (SST) was relatively small compared to prior La Niña events.
While triple-dip La Niñas have happened before, this one was notable in that it did not conform to conventional theories on how extended La Niñas develop.
Of the 13 La Niña events since 1950, only three lasted for three years. The prolonging of conditions produced by triple-dip La Niña events present globally increased risks from extreme weather.
“Compared with single-year ENSO events, triple-dip La Niñas lead to elevated risks from natural hazards like droughts, floods, heat waves and weather extremes simply because they last so long …. There were parts of the globe where it was disaster after disaster because of the extended period of cold in the tropical Pacific over these three years.”
NOAA Senior Scientist Michael McPhaden
Unsurprisingly, hot, dry and low humidity conditions resulting from rising temperatures and ENSO have led to increases in the frequency and severity of wildfires. Under certain weather conditions, these fires can easily spread out of control, threatening ecosystems, human life and property.
The 2023-24 El Niño
The 2023-24 El Niño peaked in December 2023, one of the five strongest on record.
The transition from La Niña to El Niño by the middle of 2023 caused a big swing in rainfall patterns, with many areas switching from La Niña related drought to the opposite extreme. The impacts have been dramatic, especially following the drier conditions already prevalent after the Triple La Niña.
The 2024-25 La Niña
The La Niña phase of ENSO is related to increases in the likelihood of above- and below-average precipitation over many regions of the globe. These changes in precipitation likelihoods occur during certain times of the year. Over sub-Saharan Africa, primary rainfall seasons with wet conditions are in the central and eastern Sahel (Jun-Sep) and in Southern Africa (Oct-May). Dry conditions are most likely over the Greater Horn of Africa during the Sep-Dec and Mar-May rainy seasons. Over Central Asia, dry conditions are most likely during the winter and spring precipitation seasons. In tropical South America the likelihood of wet conditions increases during Oct-Feb, with drier conditions more likely in subtropical South America.
According to the latest NOAA data (12 Sep), La Niña is favoured to emerge in the current quarter (Sep-Nov 2024), with a 71% likelihood, and is expected to persist through January-March 2025.
Appendix 3
Case Study – South America
“We are already living a scenario of an altered climate that oscillates between extreme events, either of drought or heavy rains. This has very serious consequences not only for the environment, but also for people and the economy …. I think there is a very high chance that what we are living now, the oscillation, is the new normal”
Ane Alencar, Amazon Environmental Research Institute
The recent climate extremes have serious environmental, economic and social impacts on their own.
But when such extremes are observed in combination, the resulting “compound extremes” can have devastating effects with amplified impacts – on vegetation health (e.g., tree mortality), loss of tree cover due to wildfires, human and animal health, agricultural production, infrastructure, insurance losses, GDP and so on.
South America is one of the global hotspots of these compound extremes. Over recent decades, the frequency of dry compound extremes has surged in key South American regions, including the northern Amazon. This is viewed as a direct consequence of climate change and a stronger ENSO.
The inter-annual variability of dry compounds in South America is strongly influenced by ENSO, with its phases (El Niño and La Niña) driven by the strength of trade winds.
The effects of the Triple La Niña (2020-2023) are still being felt today, after record low rainfall levels and higher temperatures for three consecutive years leading to prolonged droughts. All of this followed by a stronger than usual El Niño in 2023/24.
South America has become warmer, drier and more flammable.
Triple La Niña – Rainfall and Water levels
- In 2023, water levels in the Paraná River (the second-longest river in South America) fell to their lowest level in nearly 80 years, impacting hydropower generation, river-borne food shipments and freshwater supplies for around 40 million people in Argentina, Brazil and Paraguay.
- The three years to 2022/23 saw the lowest accumulated precipitation in Argentina’s history. In the last four months of 2022, the region received only 44% of the average precipitation – the lowest rainfall for that period in 35 years.
- Water shortages and persistent droughts were widespread in Brazil, Bolivia, Colombia, and inland Peru. In 2023, the Santo Antonio hydroelectric plant in Brazil was forced to stop operating for the first time since its start in 2012, due to drastically lowered water levels in the Madeira River.
- June–September rainfall in 2023 as well below average in much of the Amazon Basin, with 8 Brazilian states recorded their lowest July to September rainfall in over 40 years.
- The multi-year drought event and the sequence of warm and dry years in South America has contributed to the loss of 30-50% of glacier ice cover in the Andes. Smaller glaciers have fully disappeared. These glacier loses are exacerbating the water shortages, and hampering hydro-electric power generation serving hundreds of millions of people.
- There are approximately 4 000 glaciers in the Andes along the border between Chile and Argentina, with a smaller number in the tropical Andes. According to the World Glacier Monitoring Service (WGMS), the Echaurren Norte glacier in Chile has lost about 31 meters water equivalent (m w.e.) since 1975 and may disappear in the coming years.
Triple La Niña – Heatwaves and Drought
Precipitation deficits, above-average temperatures and recurrent heatwaves are responsible for the extreme droughts witnessed in recent years. This large-scale drought event results from several smaller, geographically distributed sub-events, commonly affected by a severe lack of rainfall.
Droughts often emerge as a slow onset, affecting the stability of natural ecosystems. Over the past 15 years and especially the last 4 years, droughts have become more frequent, prolonged and extreme in the region.
- The late 2022 drought put 3.5m hectares of crops and more than 18.5m cattle at risk in Argentina alone, driving economic losses of $10bn for soybean, wheat and corn producers and lowering GDP by 2%.
- By the end of March 2023, the lack of rain combined with higher-than-normal temperatures led to severe vegetation stress over Uruguay, northern Argentina, and southern Patagonia.
- During the first half of 2023, the effects of La Niña were still visible. Lack of water in the La Plata Basin hit Uruguay, northern Argentina and southern Brazil the hardest.
- In Uruguay, the summer of 2023 was the driest of the last 42 years, reducing water storage to critically low levels. Uruguayans went through the worst drought since 1947, affecting over 80% of the country, with a 60% reduction in agricultural yields and widespread water shortages.
- In Tefé Lake, in the Brazilian Amazon, water temperature reached a record high and over 150 endangered river dolphins died.
- Extreme heat and heatwaves affected central South America from August to December 2023. Brazil, Peru, Bolivia, Paraguay and Argentina all recorded their highest September temperatures.
- The intense heatwaves that swept the region increased evapotranspiration levels, reducing the amount of moisture available in soils and worsening the impacts on crops.
- As 2023 progressed and El Niño set in, drought became increasingly widespread, continuing the previous long-term drought trend.
- In the middle of the austral winter, temperatures in parts of Brazil exceeded 41 °C in August.
Drought information from CEMADEN with link
The 2024 droughts in South America
The Amazon region is presently undergoing unprecedented stress from warming, extreme drought, deforestation and fires. Several South American countries are currently experiencing their worst droughts on record, which is also fuelling a number of wildfires. The region has been facing one of the longest and largest droughts in recent decades. The effects are already evident on crop yields, the economy and ecosystems.
Atmospheric moisture from the southern Amazon is responsible for 20% of the rainfall that falls over South America. Amazon basin deforestation reached record highs in 2022 – contributing to the decrease in rainfall over South America.
The drought problem this year extends over 5 million square kilometers – 58% of the national territory. The drought’s impact is being felt in the rainforest. Areas of forest along the riverbanks, which accumulate thick layers of leaf litter, are particularly susceptible to wildfire.
- In 2024, large wildfires burned across many of the heat-stricken regions, covering as much as 60% of the Amazon region with smoke.
- Although many fires are the work of prospectors and farmers, many fires are burning in pristine areas far from active attempts at deforestation – the rainforest is getting just dry enough that it can catch fire.
- Brazil experienced several record-breaking temperatures in 2024, including a heatwave in March and August temperatures in Central Brazil and in the North that were 7°C above normal. Rio de Janeiro reached 42 C in March 2024, with its heat index reaching a record 62.3°C.
- The Rio Negro, one of the Amazon’s largest tributaries, hit a record low level since observations began in 1902 – its water levels are falling around 7 inches a day.
- The Paraguay River has hit a record low, its levels depleted by a severe drought upriver in Brazil. The depth of the river has broken the previous record low in October 2021. The national Meteorology and Hydrology Directorate expects the river will keep falling with no rain forecast. Navigation is practically halted due to the extreme drop in water levels, affecting grain and other shipments.
- The Parana River in Argentina also near year-lows
- Both rivers start in Brazil, eventually joining and flowing into the sea near Buenos Aires. They are important routes for soy, corn and other trade.
- Even with the rainy season ahead, La Nina 2024/25 is expected to bring less rain, worsening drought conditions.
Despite being a region blessed with abundant water resources, the region continues to grapple with challenges in water availability, quality and distribution.
150 million people in South America live in water-scarce areas where droughts are also very common. Traditionally water-rich areas such as the Amazon are becoming increasingly vulnerable to droughts, as seen from the devasting droughts of 2023 and 2024. Leaving millions with failed crops, water shortages, power cuts, and wildfires that threaten the already delicate ecosystem of the Amazon.
South American wildfires
As shown in the picture above (link here), a devastating series of wildfires have significantly impacted Bolivia, Brazil, Chile, Colombia, Ecuador and Peru.
Based on Global Wildfire Information System (GWIS) satellite imaging, about 346,112 wildfire hotspots have damaged or destroyed just under 84 million hectares. The massive area burned was primarily caused by climate change and the consequences of the 2023–2024 South American drought on fire conditions.
The wildfires have caused further significant deforestation of the Amazon rainforest, and also impacted several other international biomes including the Pantanal wetlands, becoming the second largest series of wildfires in the 21st century. Only the 2023–24 Australian bushfire season surpassed this.
Brazil wildfires
As of the date of writing, approx. 68,000 wildfires (as detected by GWIS) have burned an estimated 48.2 million hectares of tropical wetland in Brazil’s Pantanal in Mato Grosso do Sul, the Amazon rainforest, and the Cerrado.
The intensity and range of the wildfires were exacerbated by strong winds and the difficult terrain. It was confirmed in May that the Cerrado had suffered its worst drought in over 700 years. As a result of the fires, a massive diagonal corridor of smoke spread across the continent from Colombia to Uruguay.
Climate scientists noted that the 2024 Brazil wildfire season started earlier than typical seasons which start around July, and was also more intense due to decreased rainfall in certain regions leading to prolonged drought.
In August 2024, the Amazon had several of its rivers reach critically low water levels in the first weeks of its dry season, with several rivers in the southwest Amazon reaching their lowest point on record for their respective times of the year. The Amazon Cooperation Treaty Organization (ACTO) released a technical statement reporting that the Amazon basin had been significantly impacted by drought conditions, and anticipated that it would cause significant issues in its member states: Bolivia, Brazil, Colombia, Ecuador, Guyana, Peru, Suriname and Venezuela.
“God has cared for these trees, saved them from drought, disease, avalanches, and a thousand tempests and floods. But he cannot save them from fools”
John Muir
Brazilian President Luiz Inacio Lula da Silva pledged to stop illegal deforestation in the Amazon by 2030 to help reduce the impact of global warming.
As an aside, the fires in Southern Africa have received less news attention, but are just as dire. With its shifting fire regions and prevailing winds, there is a fire-aerosol positive feedback which not only affects the current season, but amplifies burning in the subsequent season. Fire weather season has increased by up to 40% in Africa over the past four decades.
Agriculture and Food security
- Brazil and Argentina are major players in the global food market, ranked as the #1 and #3 food exporters worldwide respectively.
- Argentinian soybean production in 2023 was 44% lower than the five year average and the lowest since 1988/89.
- 2023 overall production was better for Brazil (soybeans up, corn down) but Argentinian production was down 26% overall.
- The increased sea temperatures also reduced fishing catches in Peru and Ecuador.
Crop production may be affected further, if current trends continue. Corn yields are projected to decline 24% by 2030, while wheat could potentially see growth of about 17%.
Along with rice and soybeans, these staple crops represent almost two thirds of human food consumption. The forecast change in future yields is due to projected increases in temperature, shifts in rainfall patterns, and elevated atmospheric CO2, making it harder to grow corn in the tropics, but potentially expanding the growing range for wheat.
With the extreme drought affecting river levels and river transportation, Paraguay (the world’s No. 3 soybean exporter) is at particular risk, with roughly 80% of its grain travelling along waterways to seaports downriver. Similarly, much of Argentina’s grain goes down the Parana from Rosario. And in Brazil, with record wildfires and low water levels – soy and corn shipments in centre-west states such as Mato Grosso have been hit hardest.
Proactive drought risk management.
Some countries – Brazil, Mexico and Uruguay – have implemented regulations, policies and measures such as early warning systems and insurance mechanisms to deal with droughts.
The region as a whole needs more cross-sector coordination and planning, including investments that can help mitigate impacts and build better drought resilience.
Water resource management should be cautiously planned to limit impacts, and adaptation strategies should be urgently undertaken to build resilience against present and future climatic challenges.
The 2024-25 La Niña
As we enter October 2024, drought conditions continue to prevail, and wildfires continue to burn across much of the region. This is now the most intense and widespread drought Brazil has experienced, since records began in 1950.
With the rainy season just weeks away, the level of the Solimoes, which flows down from the Andes in Peru, is expected to drop further in coming weeks, setting another record low to top the 2023 low.
Some respite may be around the corner as the rainy season arrives. However, the onset of La Nina is expected soon, which may return things to the “new normal”.
The Amazon rainforest
The rainforest has traditionally been responsible for the massive transportation of moisture from the ocean to the interior. Its hydrological cycle maintains the crop production of South America. Providing critical moisture for agriculture and water reserves in Central Brazil, Paraguay, Uruguay and northern Argentina. Studies show that the moisture cycle even regulates rainfall patterns as far as the US Midwest.
Unfortunately, more and more of the rainforest is disappearing, through deforestation, development and now wildfires / natural disasters. This brings the rainforest within distance of its tipping point, and weakens the water transport mechanism. This will have far-reaching implications for global weather – from the Sahara and African continent, to North America, to Asia, and for global food security.
