The following article appeared online in The Conversation on 28 May, 2012. Its author is Joseph Kidston,Lecturer, Climate Change
Research Centre at University of New South Wales. Its most important point is in the final sentence: "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." It also points out that much of the global warming experienced over the last 200 years may not be due to the emissions of greenhouse gases such as CO2.
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 Chnage Research Centre at the University of NSW 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.
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.
