climate science Archives

Global warming increases drought severity

Climate science in any real sense is only about as old as I am (as per this bio of one of its founders) and it reached maturity at about the same time I did – years ago, that is. But it is still learning, as I hope I am, and still making important new discoveries. Here’s a very good example, a paper published this month in one of the world’s greatest journals, Nature.

I will quote the Abstract, the authors’ summary, in full:

Drought is one of the most common and complex natural hazards affecting the environment, economies and populations globally. However, there are significant uncertainties in global drought trends, and a limited understanding of the extent to which a key driver, atmospheric evaporative demand (AED), impacts the recent evolution of the magnitude, frequency, duration and areal extent of droughts. Here, by developing an ensemble of high-resolution global drought datasets for 1901–2022, we find an increasing trend in drought severity worldwide.

Our findings suggest that AED has increased drought severity by an average of 40% globally. Not only are typically dry regions becoming drier but also wet areas are experiencing drying trends. During the past 5 years (2018–2022), the areas in drought have expanded by 74% on average compared with 1981–2017, with AED contributing to 58% of this increase. The year 2022 was record-breaking, with 30% of the global land area affected by moderate and extreme droughts, 42% of which was attributed to increased AED.

Our findings indicate that AED has an increasingly important role in driving severe droughts and that this tendency will likely continue under future warming scenarios.

Source: Gebrechorkos, S.H., Sheffield, J., Vicente-Serrano, S.M. et al. Warming accelerates global drought severity. Nature (2025). https://doi.org/10.1038/s41586-025-09047-2

What’s that in plain language?

Most people think droughts happen simply because it stops raining. But scientists have found another powerful force at play. As the air gets warmer, it increases what’s known as atmospheric evaporative demand, or AED.

Think of AED as a giant sponge in the sky. As it heats up, it pulls more and more moisture from the land below, drying out soil, draining rivers, and stressing plants, often faster than that moisture can be replaced.

We don’t know much about whether global warming increases the likelihood of drought, but the authors found that…

…AED has increased faster than precipitation rates, suggesting an alarming tendency towards drier conditions. … In warm areas, raising the temperature by just a couple degrees can dramatically increase the atmosphere’s ability to draw moisture from crops, rangelands, and forests…

This study reinforces past work showing that droughts will become more intense in a warming world. This has implications for global food and water security, which may in turn amplify political instability and conflict. Easier to see are more direct links between increased AED and wildfire. A thirsty atmosphere desiccates plants, which contributes to larger wildfires.

How science is shared

Those two quotes are from scitechdaily, a respected science news site which takes hard science stories and makes them more accessible. The Conversation does the same, and so do several others.

Scientific discoveries eventually make their way on to the more popular media, often via such intermediaries, at the same time as they are being circulated in the science community.

Sadly, it can take a while before they are properly absorbed, as I said in Too many 1-in-100-year floods in March.

The new IPCC report (AR6) – an introduction

The IPCC is one of the world’s biggest scientific projects, with thousands of scientists in dozens of countries collaborating since 1988 to produce a series of reports. Its Sixth Assessment Report (AR6) is in three parts. Working Group 1 released Climate Change 2021: The Physical Science Basis last August, and WG2 has just released Climate Change 2022: Impacts, Adaptation and Vulnerability which “assesses the impacts of climate change, looking at ecosystems, biodiversity, and human communities at global and regional levels,” to quote its home page. Part 3 and a Summary will appear later this year.

Continue reading “The new IPCC report (AR6) – an introduction”

The Brick – a fable for our times

I wrote this parable in 2009 as a contribution to an online debate and then forgot about it for years. I recently came across it again, however, and thought it still deserved readers, so here it is. Feel free to share it.

The Brick

“G’day Tom,” said Jamie, “How’s things?”

“Not too bad, mate.”

“You’re limping, though. What happened?”

“Oh, I dropped a brick on my big toe when I was clearing up a couple of days ago. Continue reading “The Brick – a fable for our times”

Broecker: Fixing Climate

Fixing Climate
Robert Kunzig and Wallace Broecker
Profile, 2008

Fixing Climate is both interesting and useful but not in the ways that the authors intended. That’s not entirely their fault, since climate science and mitigation have changed enormously in the ten years since it was published.

The book tracks the life and work of Wallace Broecker, who was born in 1931 and was just the right age to become a pioneer and then a leader in the (then) very young field of climate history and (hence) climate change. Continue reading “Broecker: Fixing Climate”

Warning: climate change ahead

The really short version of the climate change story hasn’t changed much for the last ten years or more, of course: we know we are cooking the earth and things are going to get very uncomfortable, if not catstrophic, unless we stop pumping CO2 into the atmosphere. (NASA’s version is one of the simplest, clearest and most authoritative if you need to direct anyone to such a resource.) However, the scientific work continues to improve our knowledge every year. Two significant recent publications are discussed in not-too-technical terms in two highly regarded blogs, RealClimate and ThinkProgress.

Should we trust the bloggers? These bloggers, yes. Stefan Rahmstorf, who discusses Marcott’s paper in Science, teaches physics of the oceans as a professor at Potsdam University, is a member of the Advisory Council on Global Change of the German government and is a lead author of the paleoclimate chapter of the 4th assessment report of the IPCC. Joe Romm, who discusses Hansen’s paper, worked at the highest levels of the US Department of Energy in the 1990s before moving into full-time environmental campaigning. (For more about them, see Wikipedia: Rahmstorf, Romm.) From here on, this summary uses their words, drastically condensed; follow the links at the end for the full versions.

Marcott study: The End of the Holocene

by Stefan Rahmstorf

Recently a group of researchers from Harvard and Oregon State University has published the first global temperature reconstruction for the last 11,000 years – that’s the whole Holocene (Marcott et al. 2013). The results are striking.

Over the last decades, numerous researchers have painstakingly collected, analyzed, dated, and calibrated many data series that allow us to reconstruct climate before the age of direct measurements. Such data come e.g. from sediment drilling in the deep sea, from corals, ice cores and other sources. Shaun Marcott and colleagues for the first time assembled 73 such data sets from around the world into a global temperature reconstruction for the Holocene [i.e. the 11,700 years since the last Ice Age].

Global temperature reconstruction from proxy data by Marcott et al, Science 2013

The climate curve looks like a “hump”. At the beginning of the Holocene global temperature increased, and subsequently it decreased again by 0.7 ° C over the past 5000 years. The well-known transition from the relatively warm Medieval into the “little ice age” turns out to be part of a much longer-term cooling, which ended abruptly with the rapid warming of the 20th Century. Within a hundred years, the cooling of the previous 5000 years was undone. (One result of this is, for example, that the famous iceman ‘Ötzi’, who disappeared under ice 5000 years ago, reappeared in 1991.)

Conclusion

The curve of Marcott et al. will not be the last word on the global temperature history during the Holocene; like Mann et al. in 1998 [the famous “Hockey Stick” which was a similar temperature record for the last 1,000 years] it is the opening of the scientific discussion. There will certainly be some corrections and improvements. However, I believe that (as was the case with Mann et al.) the basic shape will prove correct: a relatively smooth curve with slow cooling trend lasting millennia from the Holocene optimum to the “little ice age”, mainly driven by the orbital cycles. At the end this cooling trend is abruptly reversed by the modern anthropogenic warming. …

Just looking at the known drivers [factors affecting climate change] and the actual temperature history shows it directly, without need for a climate model: without the increase in greenhouse gases caused by humans, the slow cooling trend would have continued. Thus virtually the entire warming of the 20th Century is due to man. This May, for the first time in at least a million years, the concentration of carbon dioxide in our atmosphere has exceeded the threshold of 400 ppm. If we do not stop this trend very soon, we will not recognize our Earth by the end of this century.

Hansen study: Climate Sensitivity is High, Burning All Fossil Fuels Would Make Most of Planet ‘Uninhabitable’

by Joe Romm, Sept 17, 2013

James Hansen, the country’s most prescient climatologist, is out with another must-read paper, “Climate sensitivity, sea level and atmospheric carbon dioxide,” … co-authored by a number of Hansen’s former colleagues at NASA. … The key findings are:

• The Earth’s actual sensitivity to a doubling of CO2 levels from preindustrial levels (to 550 ppm) — including slow feedbacks — is likely to be larger than 3–4°C (5.4-7.2°F).
• Given that we are headed towards a tripling (820 ppm) or quadrupling (1100 ppm) of atmospheric CO2 levels, inaction is untenable.
• “Burning all fossil fuels” would warm land areas on average about 20°C (36°F) and warm the poles a stunning 30°C (54°F). This “would make most of the planet uninhabitable by humans, thus calling into question strategies that emphasize adaptation to climate change.”

Burning all or even most fossil fuels would be a true scorched Earth policy.

Given that James Hansen has been right about global warming for more than 3 decades, his climate warnings need to be taken seriously. [The] whole paper is worth reading. The authors conclude:

Most of the remaining fossil fuel carbon is in coal and unconventional oil and gas. Thus, it seems, humanity stands at a fork in the road. As conventional oil and gas are depleted, will we move to carbon-free energy and efficiency—or to unconventional fossil fuels and coal?

If fossil fuels were made to pay their costs to society, costs of pollution and climate change, carbon-free alternatives might supplant fossil fuels over a period of decades. However, if governments force the public to bear the external costs and even subsidize fossil fuels, carbon emissions are likely to continue to grow, with deleterious consequences for young people and future generations.

It seems implausible that humanity will not alter its energy course as consequences of burning all fossil fuels become clearer. Yet strong evidence about the dangers of human-made climate change have so far had little effect. Whether governments continue to be so foolhardy as to allow or encourage development of all fossil fuels may determine the fate of humanity.

That’s the end of my quote from Romm’s blog, and now you know the considered opinion of the experts. But we will be all right here in Oz, of course:

Sources:

Realclimate post on Marcott: http://www.realclimate.org/index.php/archives/2013/09/paleoclimate-the-end-of-the-holocene/

Marcott’s paper on Science: http://www.sciencemag.org/content/339/6124/1198.abstract
Science 8 March 2013
Shaun A. Marcott1, Jeremy D. Shakun, Peter U. Clark, Alan C. Mix. A Reconstruction of Regional and Global Temperature for the Past 11,300 Years

ThinkProgress post on Hansen: http://thinkprogress.org/climate/2013/09/17/1892241/hansen-climate-sensitivity-uninhabitable/