What are the “Rossby waves”, which could be the cause of the floods in Pakistan and the record drought in Europe?


Extreme floods are devastating Pakistan, caused by a combination of heavy monsoon rains and melting glaciers. While Pakistan is no stranger to deadly floods, this event is particularly shocking with more than 1,100 people dead so far and several million more affected.

Pakistan’s climate chief has said a third of the country is under water – an area larger than the state of Victoria.

This Northern Hemisphere summer has seen extreme weather after extreme weather events, from record-breaking droughts in Western Europe, the United States and China, to floods in Japan and South Korea.

This raises the question of how much climate change is to blame. And, if so, is that what we should expect now?

A summer of extremes

The floods in Pakistan are the latest in a series of exceptional disasters in the northern hemisphere.

Western Europe and central and eastern China have experienced record heat waves and droughts, leading to water restrictions. These heat waves and droughts have also caused crop shortages, adding to rising food prices around the world.

China is embroiled in an energy security crisis. And Italy’s longest river flows at a tenth of its usual rate. These droughts and their significant impacts are expected to continue for the foreseeable future.

Heavy downpours caused flooding in places from Dallas in the United States to Seoul in South Korea, which saw its heaviest torrential rains in a century.

Record extreme heat was also recorded in Japan, the central United States and the United Kingdom, where temperatures exceeded 40℃ for the first time.

It has also only been a few months since we saw temperatures reaching 50℃ ahead of the monsoon rains in northern India and Pakistan.

Put it into perspective

While it is true that many of the extreme events this summer have been exceptional, we normally see more high-impact extreme weather events in the Northern Hemisphere summer than at any other time. Indeed, extreme heat, torrential rains and drought are more likely at the hottest time of the year.

Two-thirds of the planet’s land and more than 85% of the world’s population are in the northern hemisphere. This means that there are more people affected by extreme weather than in the Southern Hemisphere, making the Northern Hemisphere summer the perfect time for disasters to have serious impacts.

In addition, extreme weather events can occur at the same time in different places, due to large-scale atmospheric waves called “Rossby waves”, which are a natural phenomenon, such as La Niña and El Niño.

In 2010, western Russia experienced severe heat and wildfires, while Pakistan experienced some of its worst flooding to date. These events were connected by a Rossby wave causing a blocking high pressure pattern over western Russia and persistent low pressure over Pakistan.

Rossby waves can also result in heat waves occurring at the same time, thousands of miles away. Earlier this summer in the northern hemisphere, we saw simultaneous heat waves hitting the western United States, western Europe and China.

The Rossby waves may have contributed to simultaneous disasters this summer, but it’s too early to tell for sure.

Climate change and endless extremes

With so many extreme weather events causing mass deaths and significant economic and environmental problems, it is worth asking whether climate change can make these events worse.

Human-caused climate change has warmed the planet by about 1.2℃ so far, which has made certain types of extreme weather more frequent and intense, especially extreme heat waves and record high temperatures. .

Every heat wave in today’s climate carries the footprint of climate change resulting from our greenhouse gas emissions. Indeed, rapid analyzes have already demonstrated that the human effect on the climate has significantly increased the likelihood of extreme heat in India and Pakistan in May, and record high temperatures in the UK in July.

Research also shows that climate change increases the frequency of simultaneous heat waves in the northern hemisphere, mainly due to long-term warming.

It is less clear whether the Rossby wave pattern that causes simultaneous heat waves at different locations is becoming more common.

Climate change is also altering precipitation patterns, making drought worse in some areas, such as much of Western Europe.

And short-lived heavy downpours and extreme heavy rains, like those seen in Seoul and Dallas in recent weeks, are being intensified by climate change. Indeed, global warming allows the air to retain more moisture – for every 1℃ of warming, the atmosphere can retain 7% more moisture.

Indeed, heavy rainfall in Pakistan follows an observed trend of increasing extreme daily rainfall totals. This region of the world is expected to experience a continued intensification of extreme daily and multi-day rainfall over the summer as the planet warms.

The 5-day maximum rainfall in June-August is expected to increase in Pakistan with a global warming of 2°C. IPCC AR6 Interactive Atlas

The worst extremes to come

We can expect more extreme weather events in the coming years as global greenhouse gas emissions continue at near record rates.

Scientists have been predicting worsening extreme weather events – especially heat waves – for decades. Now we see this happening before our very eyes.

Some extreme heats in recent years have been far beyond what we thought would occur after just over 1℃ of global warming, such as the record-breaking heat of western North America last summer. But it’s hard to say whether our projections underestimate the extreme heat.

In any case, the world must prepare for other possible record high temperatures in the months, years and decades to come. We must quickly decarbonize to limit the damage caused by future extreme events.

This article first appeared on The Conversation.


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