How does a cloud physicist find himself writing a paper about the clear-sky (i.e., cloud-free) atmosphere? You see, Earth's hemispheric albedo symmetry operates in mysterious ways.
What in the world is Earth's hemispheric albedo symmetry?
"Albedo" is the scientific term for how reflective something is. Earth's hemispheric albedo symmetry is a major mystery in atmospheric science. We've observed that the Northern and Southern Hemispheres reflect identical amounts of sunlight since reliable satellite observations became available in the 1960s. However, we don't have a good theoretical understanding of why this should be the case and our state-of-the-art global climate models do not systematically reproduce the observed symmetry.
Earth's hemispheric albedo symmetry is comprised of two asymmetries: The Northern Hemisphere is brighter in clear-skies whereas the Southern Hemisphere is cloudier.
The original and most prominent explanation of Earth's clear-sky hemispheric albedo asymmetry is that the continents are mostly located in the Northern Hemisphere, and land is brighter than water, so that's why the Northern Hemisphere clear-sky is brighter. Land and ocean surfaces also have differences in the wavelengths of sunlight they reflect best. The Northern Hemisphere shows greater reflection in the near-infrared wavelengths associated with land and vegetation while the Southern Hemisphere is dominant in the visible wavelengths associated with reflection by clouds. The distribution of land and ocean is stable on very long timescales, so this explanation suggests that the clear-sky asymmetry has been a feature of Earth's climate for millions of years.
However, observations show that reflection from the atmosphere, not the surface, dominates the hemispheric albedo asymmetry. This is consistent with the greater amount of reflective aerosol, or airborne particulate matter, in the Northern Hemisphere. Some of this aerosol is natural — think of things like the desert dust from the Sahara that sometimes makes it all the way across the Atlantic Ocean to influence weather and air quality in the eastern U.S. But much of the Northern Hemisphere aerosol is the result of industrial activities. My research typically focuses on how this pollution affects clouds, but my interest was piqued by the idea that if man-made aerosol was a substantial contributor to Earth's clear-sky albedo asymmetry, then the asymmetry might be a much more ephemeral feature than generally recognized. In other words, Earth's clear-sky albedo asymmetry may have changed dramatically historically and may change again in the future.
Historical changes in Earth's clear-sky hemispheric albedo asymmetry
To look at past changes in Earth's clear-sky albedo, we use "historical" simulations from the latest generation of global climate models with estimates of aerosol and greenhouse gas emissions from 1850 to the present. We also use special runs (named "hist-piAer") in which greenhouse gas emissions increase over time with industrialization but aerosol emissions remain at their pre-industrial levels to isolate the effects of aerosol pollution. Figure 1 shows the average model response. Aerosol emissions drive a large increase in the albedo asymmetry, especially between 1960 and the present-day. This time period also happens to be when we have reliable satellite estimates of Earth's albedo, meaning we may only have observations starting from a relatively unusual time for Earth's albedo asymmetry.
Future changes in Earth's clear-sky hemispheric albedo asymmetry
To look at future changes, we use three scenarios of future emissions: one "sustainability" pathway with decreasing emissions of greenhouse gases and aerosol (called SSP1-2.6), a "middle-of-the-road" pathway (SSP2-4.5), and a "regional rivalry" pathway in which countries do not agree on emissions cuts and both greenhouse gas and aerosol emissions remain high (SSP3-7.0). Our results show that the clear-sky asymmetry is set to decrease substantially over the next few decades regardless of which emissions trajectory we take. How is that so?
In the "sustainability" scenario, dramatic declines in aerosol emissions from burning fossil fuels causes the atmospheric component of the albedo asymmetry to shrink. In the high-emissions "regional rivalry" scenario, however, the atmospheric component of the albedo symmetry does not change very much while the surface component shrinks a lot. Figure 2 summarizes these results. The middle-of-the-road scenario, fittingly, falls between the low- and high-emission results.
So, why would the surface component decline in a warming world? It's not that rising oceans swamp the land like in a B movie. Instead, it's because Arctic sea ice and land-based snow and ice in the Northern Hemisphere disappear much faster than Antarctic sea ice. It turns out that ice plays a big role even in the present-day: the Antarctic ice sheets are so much more reflective than the Arctic that they cancel out most of the greater reflection from the Northern Hemisphere continents.
One caveat with these results is that the observed breakdown of reflected sunlight from NASA's Clouds and the Earth's Radiant Energy System (CERES) satellite instruments between the surface and atmosphere is very different than that simulated by the models, with observations showing a much larger atmospheric component and smaller surface component. For that reason, and because of greater natural variability in sea ice, we may have more confidence in the atmospheric declines if aerosol emissions decrease than the surface declines with unchecked warming.
Why should we care?
We can expect to start seeing changes in the clear-sky asymmetry within the next few decades. If the observed symmetry in all-sky reflection is just a fluke, we would find out soon as the Southern Hemisphere becomes noticeably brighter than the Northern Hemisphere. However, if clouds adjust to maintain all-sky symmetry in the face of clear-sky changes (which has not yet been proven), there could be large consequences for Earth's energy balance and hydrological cycle. In a worst-case scenario, dimming in the Northern Hemisphere clear-skies would lead to a reduction in Southern Hemisphere cloudiness and an acceleration of global warming. If the adjustment mechanism works via changes in tropical clouds, there could be changes in rainfall that affect vulnerable regions like the Sahel in northern Africa. If the adjustment mechanism instead works via changes in higher latitude clouds, there could be implications for the general circulation of the atmosphere and ocean and for long-term heat uptake in the Southern Ocean. Unfortunately, until we understand the mechanism by which clouds adjust to maintain Earth's hemispheric albedo symmetry (or find that such a mechanism does not exist), we can only speculate about the consequences.