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Poleward-shifting climate zones - where are they headed and why?

It’s not just greenhouse gases that affect climate: other air pollution could be changing atmospheric circulation. NASA Goddard Photo and Video

The Earth’s principal climatic zones appear to be shifting poleward. If this continues, as climate models project, the weather patterns that give rise to deserts in the subtropics, and stormy wet weather in the mid-latitudes, will move towards the poles of the Earth.

These shifts are the cause of many of the future regional changes scientists expect to affect our climate.

Surprisingly, despite the fact that most models of atmospheric circulation produce these changes, the underlying causes and the precise dynamics that give rise to these poleward shifts are still not clear. This is a major focus of research in atmospheric science. One point of contention has been that models appear to consistently underestimate the shift when compared with observations.

The reason that the underlying causes are difficult to determine is that many climate “forcings”, such as increasing greenhouse gases (GHGs), changes in aerosols and increases and decreases in ozone (depending on where the change is located) all result in similar circulation changes in models of the atmosphere.

Moreover, the fluid dynamics that lead from forcings to circulation changes are largely resolved by the models, rather than prescribed. This means that the models are not “told what to do”, but rather that they simulate a fluid flow and there can be many steps between initial cause and final effect, which are difficult to understand.

Shedding some light on circulation changes

Last week a team including including Professor Steve Sherwood at the Climate Change Research Centre at UNSW published new research in the journal Nature. This work has shed some light on the underlying cause of these circulation changes. They find that in global climate models, changes in black carbon and tropospheric ozone can be more effective at causing the expansion of the tropics than well-mixed green house gases.

Black carbon particles are released through burning fossil fuels and bio fuels. You may think of them as “soot”. The input of these into the atmosphere has increased significantly over the past few decades, particularly from Asia and Africa. The sources are numerous, and include forest burning, bio fuels used for residential heating and cooking, and diesel engines. Tropospheric ozone increases have also been attributed to anthropogenic pollution. Motor vehicle exhausts and industrial processes including the generation of chemical solvents all release chemicals that increase ozone in the troposphere.

The result could be a major step toward reconciling the discrepancies between models and observations, increasing our understanding of the climate system.

The study raises the prospect that anthropogenic emissions other than GHGs will dominate global scale circulation changes, at least in the medium-term. This raises the question of whether such understanding may warrant a case for controlling emissions of ozone and black carbon.

Should we try to control black carbon’s contribution to atmospheric circulation? Jean-Etienne Minh-Duy Poirrier

Future changes are uncertain

The study also raises important questions for producing accurate regional climate projections

Large-scale circulation changes are the foundation of many regional climate changes. If these circulation changes are substantially affected by emissions of black carbon and ozone precursors, we need accurate projections of future concentrations of these substances.

Unlike well-mixed greenhouse gases, these substances remain in the atmosphere for only a short period of time. This means that their concentration at a given point in time depends on activities over just the preceding few years. This makes prediction inherently less certain than for the concentration of well-mixed greenhouse gases alone.

There is a second, and more serious reason, why future changes in large-scale circulation are extremely uncertain.

Recent work, led by Adam Scaife at the UK Met Office and Michael Sigmond at the University of Toronto, has shown that the circulation changes in a model can be critically dependent on aspects of the atmosphere that are often dismissed as unimportant.

The wind-speed in the stratosphere and mesosphere - remote parts of the upper atmosphere containing less than 10% of the atmospheric mass - is crucial to modelling circulation changes in the lower atmosphere. Unfortunately, most models are a long way from simulating the upper atmosphere accurately.

Moreover, some of the processes that determine the state of this part of the atmosphere are simply prescribed in the models. This means that important forces affecting this part of the atmosphere are approximated, or parameterized, rather than generated through the equations of fluid motion, and so it is very hard to know how they may change in the future.

So, while recent research significantly increases our understanding of the climate system, it also shows that both the forcings, and the processes that give rise to large-scale circulation changes, may be a lot less certain than we previously thought.

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