Climate

by Joe Romm CLIMATE PROGRESS

CREDIT: Shutterstock

A deeply flawed new study tries to challenge the basic scientific understanding that “wet areas are likely to get wetter and dry areas drier in a warmer world.” This Nature article has been justifiably criticized for both its methodology and for ignoring a vast literature that contradicts it.

“It is sad for the science,” when “papers like this gets published and get attention,” according to Kevin Trenberth, a leading expert on how climate change impacts the hydrological cycle. “This paper does some useful things but … the extrapolations, model evaluations and conclusions are not justified!”

Let’s start with the basic science — and the basic reason I’m surprised this article made it through peer review and got published in Nature. The study “Northern Hemisphere hydroclimate variability over the past twelve centuries” opens [including footnote numbers]:

Global warming is expected to strongly influence the hydrological cycle [2, 3, 4, 18, 19]. Model simulations and theory suggest an intensification of the wet–dry contrast at lower latitudes [5] and a widespread expansion of dry areas over land[3]. These projections have yet to be robustly detected in recent instrumental observations [8, 9].

You can find the references here. Now it is true that you can find a couple of (controversial) studies that say we can’t yet detect dry areas getting drier or an intensification of the hydrological cycle. But you can easily find vastly more studies documenting that we can and have. They were ignored by the authors here — sometimes willfully — and I’m surprised that got through peer review.

A Deeply Flawed Study

How easy is it to find a study contradicting the authors? Consider their own footnote 19 which they cite to support the opening statement “Global warming is expected to strongly influence the hydrological cycle.” Footnote 19 is “Dai, A. Increasing drought under global warming in observations and models. Nature Clim. Change 3, 52–58 (2012).”

The opening sentence of the abstract of that 2012 paper by Aiguo Dai is “Historical records of precipitation, streamflow and drought indices all show increased aridity since 1950 over many land areas”!

D’oh! How exactly can you cite an article that is explicitly about “Increasing drought under global warming in observations,” that states there are lots of historical records showing an intensification of the hydrological cycle and aridity, that itself links to two major review articles of those records — but then two sentences later assert “These projections have yet to be robustly detected in recent instrumental observations”?

By the way, Footnote 19 is Kevin Trenberth’s 2011 paper, “Changes in precipitation with climate change” (online here). That paper itself explicitly states: “Evidence is building that human-induced climate change (global warming), is changing precipitation and the hydrological cycle, and especially the extremes. This article first discusses the observed changes….”

Trenberth runs through the considerable literature available on this subject even five years ago and explains: “Drought has also generally increased throughout the 20th century (Dai et al. 2004, Trenberth et al. 2007a), as measured by the Palmer drought severity index (PDSI). Dai et al. (2004) show that very dry land areas across the globe (defined as areas with PDSI less than –3.0) have more than doubled in extent since the 1970s.

It’s one thing for the authors of this new Nature paper to decide they don’t agree with any of these studies — and instead side with a couple of studies that disagree with them. But it’s an entirely different — and embarrassing — thing to simply ignore them and pretend as if they don’t exist.

Moreover, there are a lot of other recent studies that come to a very different conclusion, perhaps most notably the comprehensive analysis “Significant anthropogenic-induced changes of climate classes since 1950” published last summer. That study concluded:

About 5.7% of the global total land area has shifted toward warmer and drier climate types from 1950–2010, and significant changes include expansion of arid and high-latitude continental climate zones…. we find that these changes of climate types since 1950 cannot be explained as natural variations but are driven by anthropogenic factors.

I’m sure the editors at Nature were familiar with this study because they published it themselves! That study ran a sensitivity test whose “results suggest that significant changes in major climate types are robust and consistent among different datasets.”

The Key Role Warmer Temperatures And Evaporation Play In Extreme Drought

Significantly, that study concluded “rising temperature and decreasing precipitation are about equally important in causing the expansion of semiarid climate in Asia and western North America….”

The reason that is significant is most climate models underestimate the impact of global warming on soil moisture and drought because they only look at the contribution of changes to precipitation. Most climate models do not examine the equally important impact of warming temperatures, which all by itself increases evaporation, dries out soil, and worsens drought even when precipitation hasn’t changed at all.

And this neglect of the contribution of evaporation — which I and others have written about extensively — appears to be yet another flaw in the new Nature piece. Since the researchers don’t have access to actual data on precipitation over the past several hundred years, they have to use so-called paleoclimate “proxy data,” such as tree rings.

But as one leading expert on this subject, Dr. Markus Donat, explained, while the proxy data of water availability “is affected by both precipitation and evaporation” they “compare against modeled precipitation only.” The result is that “the researchers’ claim of discrepancies between climate models and proxies during the most recent century is based on an apples-to-oranges-comparison.” Donat explains:

The model simulations show increased precipitation over the past 100 years, consistent with an intensification of the hydrological cycle as temperatures rise related to increasing greenhouse gases in the atmosphere. With increasing temperatures, however, also the amount of evaporated water may increase, and this may compensate the increases in precipitation. This means that the water availability proxies and modelled precipitation are not reasonably comparable over the last century (which is the only period where their comparison seems to point to discrepancies between proxies and models).

In a recent study [Donat et al., 2016, Nature Climate Change] we found that, when aggregating over the dry and wet regions of the world, precipitation changes are consistent between models and observations over the past 60 years.

D’oh! The Nature Climate Change study Donat led, “More extreme precipitation in the world’s dry and wet regions” concluded “extreme daily precipitation averaged over both dry and wet regimes shows robust increases in both observations and climate models over the past six decades.”

So yet another “robust” finding that directly contradicts a central point of this study.

It is perhaps no surprise that one of the country’s leading climatologists, Dr. Michael Mann, concluded his critique of the study:

So, in conclusion, it would be rather dangerous to extrapolate from this one potentially flawed new paleoclimate study any sweeping conclusions about climate models and human-caused climate change. Such over-interpretations of paleoclimate data poorly serve the critical public discourse over the impacts of climate change, and can in fact do harm to the paleoclimate discipline by publicizing bold but unsubstantiated claims that are very likely to be refuted by further work.

The Future Is Hot, Dry, and Dust-Bowlified, If We Don’t Act ASAP

As for the future, climate models will not likely overestimate future extremes of deluges and, especially, droughts, as this flawed study suggests. In fact, it is almost certain that most climate models have been underestimating future drought extremes. Again, that’s because most models have been focusing on changes in precipitation — rather than on changes in both precipitation and evaporation.

In 2014, I wrote about a study that described itself as “one of the first to use the latest climate simulations to model the effects of both changing rainfall and evaporation rates on future drought.” The Columbia University news release, headlined “Warming Climate May Spread Drying to a Third of Earth, Says Study” explains the conclusions:

The study estimates that 12 percent of land will be subject to drought by 2100 through rainfall changes alone; but the drying will spread to 30 percent of land if higher evaporation rates from the added energy and humidity in the atmosphere are considered. An increase in evaporative drying means that even regions expected to get more rain, including important wheat, corn and rice belts in the western United States and southeastern China, will be at risk of drought.

Last year, a National Center for Atmospheric Research (NCAR) study concluded that “the weather patterns that typically bring moisture to the southwestern United States are becoming more rare, an indication that the region is sliding into the drier climate state predicted by global models.” And that study only analyzed data through 2010 — so it didn’t even include the devastating drying seen recently in large parts of the Southwest, such as the worst drought California has seen in over a thousand years!

Since climate scientists were right about this drying trend we must take seriously their current projections of widespread global megadroughts in the coming decades on our current CO2 emissions pathway — including in the U.S.’s own breadbasket.

Here is a 2015 NASA projection of what the normal climate of North America will look like unless we keep taking stronger and stronger measures to slash carbon pollution. The darkest areas have soil moisture comparable to that seen during the 1930s Dust Bowl.

I called this prolonged, multi-decadal warming and drying “Dust-Bowlification” in a 2011 Nature review article, “The Next Dust Bowl,” because the 1930s Dust Bowl seems to be the best analogy to what’s coming. But in fact, the coming multidecadal megadroughts will be much worse than the Dust Bowl of the 1930s — “worse than anything seen during the last 2000 years,” as explained in a major 2014 study, “Assessing the risk of persistent drought using climate model simulations and paleoclimate data.” They will be the kind of megadroughts that in the past destroyed entire civilizations.

If the world does manage to keep total warming well below 2°C (3.6°F), then we can minimize the chances of catastrophic mega-droughts. But only if we heed the overwhelming scientific literature on the subject.