An Observational Constraint on Hydrologic Cycle Intensification: Frequently Asked Questions

This page answers questions about a study titled “An Observational Constraint on Hydrologic Cycle Intensification,” authored by Anthony DeAngelis, Xin Qu, and Alex Hall at UCLA and Mark Zelinka at Lawrence Livermore National Laboratory. The paper was published as a research letter in Nature on December 10, 2015. Read the full study

Why is this study important?

Climate change is expected to change the water cycle in the future. On average, global precipitation is expected to increase. But different regions are likely to be affected very differently: Regions that are currently wet will get wetter, and dry regions will get drier. Global climate model projections of future precipitation are the basis for understanding what will happen to precipitation on regional and local scales.

Currently, different global climate models give very different answers about how much average global precipitation will increase. This makes it difficult to have confidence in the model projections of this metric, and to use them to understand what will happen to precipitation in any given region.

What is the main finding of this study?

We found an explanation for why future precipitation projections vary so much from model to model. It turns out that models disagree on how much extra sunlight the atmosphere absorbs as temperatures rise and the amount of water vapor in the atmosphere increases. This in turn leads them to disagree on how much precipitation occurs in the future.

We found that many global climate models underestimate the sunlight absorbed in the atmosphere, and as a result these models tend to overestimate the precipitation increase that occurs in association with climate change.

What are the implications of this finding?

This finding identifies an aspect of global climate models that could be improved. If models can more accurately represent increases in sunlight absorbed by the atmosphere, they are more likely to be accurate about future global precipitation changes.

If this aspect of the global climate models were improved, we estimate the average model’s projection of how much precipitation will increase with every increment of atmospheric warming would be about 20% smaller* than it is now. More accurate global projections of future precipitation would allow scientists to make more accurate projections about precipitation, droughts, and floods at regional and local scales.

What are global climate models, and why don’t they agree on future changes?

Global climate models are powerful, highly complex computer models that simulate the atmospheric and oceanic processes that govern Earth’s climate. Climate scientists use them to project future climate under different scenarios of emissions of greenhouse gases, which include carbon dioxide and other gases that trap heat in the atmosphere. Models are constructed of many blocks of computer code representing different processes within the climate system; such as the absorption of heat emitted from the Earth’s surface by human-emitted greenhouse gases in the atmosphere, and the absorption of sunlight by atmospheric water vapor.

There are more than 30 different global climate models, developed at scientific institutions around the world. Many physical aspects of the climate system can be represented in different ways, and different developers take different approaches. Global climate models are constantly being tested and refined; climate scientists and model developers fine-tune them as they make discoveries about the limitations of current model construction and the intricacies of the climate system. Studies like this one are part of that process.

If global climate models don’t get sunlight absorption right, should we doubt other aspects of them?

Global climate models are the best tools we have for projecting future climate, but they are not perfect and are constantly being improved. This study is part of that process.

Climate scientists rigorously test global climate models, such as by using them to simulate the climate of a past time period and comparing these simulations to the historical record. As a result of these and other types of tests, scientists have enormous confidence in many aspects of global climate models. One such aspect is how the models represent the main driver of climate change: the absorption of heat emanating from the Earth’s surface by human-emitted greenhouse gases. Sunlight isn’t part of this mechanism, because human-emitted greenhouse gases such as carbon dioxide virtually ignore sunlight.

However, sunlight is absorbed by water vapor, and as this study shows, it does play a role in precipitation. When more sunlight is absorbed in the atmosphere, less precipitation occurs. (See the answer to the next question to learn why.)

Our study shows that the representation of sunlight absorption is a necessary area of improvement for global climate models, but it does not imply that other key aspects of the models should be doubted.

Some model developers are already actively working on improving how models represent sunlight absorption. That the issue has not yet received wider attention so far may be because until now, it was not shown to be an important factor in precipitation changes. Developers have focused extensively on causes of model differences in warming, and have improved how global climate models represent these factors.

Why would increased absorption of sunlight in the atmosphere lead to less precipitation?

A basic answer to this question is that cloud development and precipitation are the result of an unstable atmosphere, in which temperatures closer to the earth’s surface are hotter than temperatures higher up in the atmosphere. When more sunlight is absorbed by water vapor in the atmosphere, it has the effect of stabilizing the atmosphere: There’s less of a difference between temperatures higher up and temperatures lower down. As a result, fewer clouds are formed, and less precipitation occurs.

The mechanism we investigated in our study is a bit more complex. We analyzed the influence of sunlight absorption on precipitation by considering the sources and sinks of energy at Earth’s surface and in the atmosphere. This analysis is laid out in more detail in the paper and in an associated commentary, published in the same Nature issue, by Steven Sherwood of the University of New South Wales in Australia.

Is this the first study to investigate why global climate models disagree on future global average precipitation?

Ours is not the first study to investigate this question, but to our knowledge it is the most comprehensive. We investigated several possible explanations for the disagreement before concluding that sunlight absorption was the most important one. Our study is unique in that we also analyzed satellite observations to show that many of global climate models are in error in their estimates of sunlight absorption.

Who conducted the study, and how was it funded?

The work was undertaken as part of a collaborative project between Lawrence Livermore National Laboratory and UCLA called “Identifying Robust Cloud Feedbacks in Observations and Models,” funded by the US Department of Energy. The lead author of the study is Anthony DeAngelis, a postdoctoral researcher working with Professor Alex Hall in the Department of Atmospheric and Oceanic Sciences at UCLA.

Read the full study

Learn more about our group's research

*An earlier version of this page listed this estimate as 40%, but 20% is the correct estimate. For more information, see the corrigendum to the full study.