The role of baroclinic activity in controlling Earth’s albedo in the present and future climates

Abstract

Clouds are one of the most influential components of Earth’s climate system. Specifically, the midlatitude clouds play a vital role in shaping Earth’s albedo. This study investigates the connection between baroclinic activity, which dominates the midlatitude climate, and cloud-albedo and how it relates to Earth’s existing hemispheric albedo symmetry. We show that baroclinic activity and cloud-albedo are highly correlated.

By using Lagrangian tracking of cyclones and anticyclones and analyzing their individual cloud properties at different vertical levels, we explain why their cloud-albedo increases monotonically with intensity. We find that while for anticyclones, the relation between strength and cloudiness is mostly linear, for cyclones, in which clouds are more prevalent, the relation saturates with strength. Using the cloud-albedo strength relationships and the climatology of baroclinic activity, we demonstrate that the observed hemispheric difference in cloud-albedo is well explained by the difference in the population of cyclones and anticyclones, which counter-balances the difference in clear-sky albedo.

Finally, we discuss the robustness of the hemispheric albedo symmetry in the future climate. Seemingly, the symmetry should break, as the northern hemisphere’s storm track response differs from that of the southern hemisphere due to Arctic amplification. However, we show that the saturation of the cloud response to storm intensity implies that the increase in the skewness of the southern hemisphere storm distribution toward strong storms will decrease future cloud-albedo in the southern hemisphere. This complex response explains how albedo symmetry might persist even with the predicted asymmetric hemispheric change in baroclinicity under climate change.

The role of baroclinic activity in controlling Earth’s albedo in the present and future climates

If you look at Earth from space, it appears to be uniformly bright, even though the Southern Hemisphere has more dark ocean waters that should reflect less sunlight. This is unexpected since the reflection of sunlight should vary based on the amount of dark water in the Southern Hemisphere.

Scientists have been puzzled by this mystery since the 1970s when satellites were first used to measure reflected solar radiation, known as albedo. However, the mystery may have finally been solved.

The explanation is that although the oceans in the Southern Hemisphere absorb more sunlight, they also produce more storms. The resulting storm clouds act as reflectors that balance out the solar radiation being sent back into space.

This discovery is significant since it helps us better understand Earth’s radiation balance and its effectors. Yohai Kaspi, a geophysicist at the Weizmann Institute of Science in Israel, explains that the new research solves a basic scientific question.

While there are many factors that affect Earth’s albedo, such as snow on the ground, measurements closer to the surface show a difference in albedo between the hemispheres.

However, when viewing Earth from a greater distance, the uniform brightness is no longer the case.

To solve the mystery, the researchers gathered data from multiple sources such as the NASA Terra satellite and the ERA5 global weather dataset. These datasets incorporated satellite readings and information on cloud cover, storm location, and intensity from 50 years of data.

By analyzing the patterns of cyclones that generate clouds and anticyclones that suppress clouds formed by the interaction of temperature and pressure in the atmosphere, the scientists were able to demonstrate how Earth’s albedo is balanced out.

Or Hadas, a climate scientist from the Weizmann Institute of Science, explains that strong storms above the Southern Hemisphere generate cloud albedo, which acts as a high-precision offsetting agent to the large land area in the Northern Hemisphere, thus preserving symmetry. This links storms with the brightness of Earth’s surface and that of clouds, solving the symmetry mystery.

However, a question remains regarding how global warming may affect this balance. Climate models predict that as the planet warms, the Northern Hemisphere will have fewer overall storms, while the Southern Hemisphere will have fewer weaker and moderate storms due to Arctic amplification.

This change in weather patterns could potentially unbalance the albedo symmetry. However, the researchers’ data suggests that severe weather events might not contribute to more cloud-albedo over the Southern Hemisphere as cloud levels reach saturation in strong storms.

A Southern Hemisphere skewed towards stronger storms with fewer storms overall might lead to a decline in albedo over both hemispheres, maintaining symmetry.

However, it is currently challenging to predict what will happen to Earth’s overall shine in the face of global warming.

Yohai Kaspi explains that it is not yet possible to determine whether the symmetry will break due to global warming.

As global warming continues, geoengineered solutions will become necessary for human life to coexist with it. A better understanding of basic climate phenomena, such as the hemispheric albedo symmetry, could aid in developing these solutions.

Orginal Article: The role of baroclinic activity in controlling Earth’s albedo in the present and future climates

https://www.pnas.org/doi/10.1073/pnas.2208778120