Heat and Rainfall Extremes are Increasing at an Unprecedented Rate in Observational Data

Scientists discovered a 90-fold rise in the frequency of monthly heat extremes in the last 10 years compared to 1951-1980 in observation data. According to their findings, so-called 3-sigma heat events, which differ significantly from what is usual in a specific location, currently, affect around 9% of all land area at any one moment.

On average, 1 in every 4 rainfall records in the recent decade may be ascribed to climate change, which rose in a non-linear fashion. Extreme occurrences connected to human-caused climate change are already at historic levels, according to the experts, and they must be expected to rise significantly more.

For extreme extremes, what we call 4-sigma-events that have been virtually absent before, we even see a roughly 1000-fold increase compared to the reference period. They affected about 3 percent of global land area in 2011-20 in any month.

Alexander Robinson

Raindrop concentrations range from 100 to 1,000 per cubic meter (3 to 30 per cubic foot), with drizzle droplets being more frequent. Raindrops seldom have diameters more than 4 mm because they break up as they become larger. As diameters get larger, the concentration tends to decrease.

It does not decrease visibility as much as drizzle, unless when it is thick. Rain is classified by meteorologists based on its rate of fall. Light, moderate, and heavy rain had hourly rates of less than 2.5 mm, 2.8 to 7.6 mm, and more than 7.6 mm, respectively.

“For extreme extremes, what we call 4-sigma-events that have been virtually absent before, we even see a roughly 1000-fold increase compared to the reference period. They affected about 3 percent of global land area in 2011-20 in any month,” says lead-author Alexander Robinson from Complutense University of Madrid, Spain, and Potsdam Institute for Climate Impact Research, Germany.

“This confirms previous findings, yet with ever-increasing numbers. We are seeing extremes now which are virtually impossible without the influence of global warming caused by greenhouse gas emissions from burning fossil fuels.” The term ‘sigma’ refers to what scientists call a standard deviation.

For example, in 2020, both Siberia and Australia had lengthy heat waves, causing to the rise of severe wildfires in both locations. Both incidents prompted the local government to declare a state of emergency. In 2021, sections of the United States and Canada experienced life-threatening temperatures, with temperatures reaching almost 50°C.

Globally, record-breaking heat extremes rose the greatest in tropical regions, which typically have little monthly temperature fluctuation. However, as temperatures rise, record-breaking heat will become more regular in the mid-and high-latitude areas.

1 in 4 rainfall records is attributable to climate change

Rainfall totals have also grown on a daily basis. The frequency of wet records rose by roughly 30% when compared to what would be expected in a climate without global warming. This means that one out of every four records may be attributed to human-caused climate change. The Clausius-Clapeyron relation, which indicates that air can store 7% extra moisture every degree Celsius of warming, is the science behind this.

Importantly, rainfall records have been reduced in already-dry places like as western North America and South Africa, while moist regions such as central and northern Europe have witnessed a significant rise. Increasing rainfall extremes does not, in general, assist to ameliorate drought conditions.

Small temperature increase, disproportionally big consequences

When compared to the already intense preceding decade of 2000-2010, the new statistics reveal that the land area hit by 3-sigma heat extremes has approximately doubled. The 4-sigma occurrences, which are extremely strong deviations that were previously virtually nonexistent, have recently appeared in the data.

In the previous ten years, rainfall records have grown by another 5%. The relatively modest amount of warming in the last 10 years, 0.25°C, has so significantly increased climatic extremes.

“These data show that extremes are now far outside the historical experience. Extreme heat and extreme rainfall are increasing disproportionally,” says co-author Stefan Rahmstorf, from the Potsdam Institute for Climate Impact Research.

“Our analysis confirms once again that for the impacts of global heating on us humans, every tenth of a degree matters.”