Aerosols Should Mean More Warming in the South

This is just one of dozens of responses to common climate change denial arguments, which can all be found at How to Talk to a Climate Sceptic.

Objection:

Scientists claim that Global Warming from GHG's is actually being countered somewhat by Global Dimming from aerosol pollution. They even say that's what caused cooling in mid-century. But, GHG's are evenly mixed around the globe while aerosols by far impact the northern hemisphere where they are produced more than the southern hemisphere. This means that warming should be greater in the southern hemisphere, which is the opposite of what is happening. Clearly climate scientists do not know what is really going on.

Answer:

Aerosol cooling does indeed affect the northern hemisphere, where they are mostly produced, to a greater extent than the southern hemisphere. It is also correct that GHG's are well mixed in the atmosphere and apart from a lag of a very few years, the southern hemispheric concentrations are rising just the same as the northern.

Where this argument goes wrong however is in its assumption uniform CO2 concentrations imply uniform heating. In fact, it is completely in line with model expectations that CO2 dominated warming will have a disproportionately large effect in the north. The reasons lie in those complications plaguing the climate system that everyone is so fond of pointing out when it suits the argument and ignoring when it doesn't. This particular complication is understood well enough to explain what we observe.

GHG forcing has a greater effect over land, largely because of the ocean's ability to absorb far more heat without warming nearly as much and also its ability to distribute it quickly in the upper layers via convection, moving the heat into lower waters rather than greatly warming the upper waters. On land, most of this extra heat is transferred directly to the air thus showing up immediately as greater atmospheric warming. Now it just so happens that there is a disproportionate amount of land in the northern hemisphere. The land area of North America, Europe, Asia, the Middle East, most of Africa and the top of South America are all in the north, while the south has Antarctica, Australia and the rest of South American and Africa.

Another factor is the differing ocean dynamics between the north and south resulting in yet more heat mixing into deeper waters of the Southern Ocean. Sea ice feedbacks in the arctic are also much greater than in the Antarctic. There is also an expected polar amplification that is modelled and observed to occur in the Arctic. Real Climate has a good article on this.

So in short, it is simply incorrect to say that we should see now, or expect to see later more warming in the Southern hemisphere. There are, by all means, mysteries left to solve, but the greater warming in the northern hemisphere is not one of them.

This is just one of dozens of responses to common climate change denial arguments, which can all be found at How to Talk to a Climate Sceptic.

"Aerosols Should Mean More Warming in the South" was first published here, where you can still find the original comment thread. This updated version is also posted on the Grist website, where additional comments can be found, though the author, Coby Beck, does not monitor or respond there.