NASA data suggest future may be rainier than expected

June 12, 2017, NASA
Tropical rainfall may increase more than previously thought as the climate warms. Credit: teresaaaa, CC BY-ND 2.0

A new study suggests that most global climate models may underestimate the amount of rain that will fall in Earth's tropical regions as our planet continues to warm. That's because these models underestimate decreases in high clouds over the tropics seen in recent NASA observations, according to research led by scientist Hui Su of NASA's Jet Propulsion Laboratory in Pasadena, California.

Wait a minute: how can fewer clouds lead to more rainfall? Globally, rainfall isn't related just to the clouds that are available to make rain but also to Earth's "energy budget"—incoming energy from the sun compared to outgoing heat energy. High-altitude tropical clouds trap heat in the atmosphere. If there are fewer of these clouds in the future, the tropical atmosphere will cool. Judging from observed changes in clouds over recent decades, it appears that the atmosphere would create fewer in response to surface warming. It would also increase tropical rainfall, which would warm the air to balance the cooling from the high cloud shrinkage.

Rainfall warming the air also sounds counterintuitive—people are used to rain cooling the air around them, not warming it. Several miles up in the atmosphere, however, a different process prevails. When water evaporates into water vapor here on Earth's surface and rises into the atmosphere, it carries with it the heat energy that made it evaporate. In the cold upper atmosphere, when the water vapor condenses into liquid droplets or ice particles, it releases its heat and warms the .

The new study is published in the journal Nature Communications. It puts the decrease in high tropical cloud cover in context as one result of a planet-wide shift in large-scale air flows that is occurring as Earth's surface temperature warms. These large-scale flows are called the atmospheric general circulation, and they include a wide zone of rising air centered on the equator. Observations over the last 30 to 40 years have shown that this zone is narrowing as the climate warms, causing the decrease in high clouds.

Su and colleagues at JPL and four universities compared climate data from the past few decades with 23 climate model simulations of the same period. Climate modelers use retrospective simulations like these to check how well their numerical models are able to reproduce observations. For data, the team used observations of outgoing thermal radiation from NASA's spaceborne Clouds and the Earth's Radiant Energy System (CERES) and other satellite instruments, as well as ground-level observations.

Su's team found that most of the climate models underestimated the rate of increase in precipitation for each degree of surface warming that has occurred in recent decades. The models that came closest to matching observations of clouds in the present-day climate showed a greater precipitation increase for the future than the other models.

Su said that by tracing the underestimation problem back to the models' deficiencies in representing tropical high clouds and the atmospheric general circulation, "This study provides a pathway for improving predictions of future precipitation change."

Explore further: Thin tropical clouds cool the climate

More information: Hui Su et al. Tightening of tropical ascent and high clouds key to precipitation change in a warmer climate, Nature Communications (2017). DOI: 10.1038/ncomms15771

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6 comments

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Bart_A
1.4 / 5 (10) Jun 12, 2017
Given the accuracy of past climate models, I take this new one with a grain of salt.
There may be more rain.
Or less.
Or mabe the same.

Take your pick.

PTTG
4.4 / 5 (8) Jun 12, 2017
So are you a paid promoter of FUD, or is this just a hobby for you?
MR166
1 / 5 (6) Jun 12, 2017
https://phys.org/...ics.html

What a science! More rain, less rain, more hurricanes, fewer hurricanes and the list is endless.

It is great how they are able to predict what happened yesterday.
Osiris1
not rated yet Jun 13, 2017
Nice to know that the models are being refined as real world data intrudes on some folks midnite reveries. However, common horse sense says that as the world warms, more water will evaporate IAW the global higher heat budget.... and fall as rain when it cools in the thickened upper atmosphere. Seen another way, most climate models of the dinosaur ages show higher oxygen levels due to increased photosyntheses from higher plant energy budgets as well. Dinos had nostrils too small to feed their large bodies if they were to exist today...further evidence of more O2 in the remote past. Rain forests proliferated in many more places in the past as shown by archaeological studies in places as far north as today's Montana. Temperate climates even existed at the poles..
Dingbone
Jun 13, 2017
This comment has been removed by a moderator.
Zzzzzzzz
3.3 / 5 (3) Jun 13, 2017
So are you a paid promoter of FUD, or is this just a hobby for you?

He is a fecal regurgitator. Its not a hobby, its an addiction. A filthy one at that.....

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