Sulfur emissions may have slowed temperature rise
A
paper published in the Proceedings of the National Academy of
Sciences is attracting attention in the
media and across the blogosphere today.
Despite the clear long-term trend in global temperatures showing the planet
is warming, a slowing in the long term warming trend over
the past decade has led some to conclude that "global warming has
stopped". While Met Office data shows that this claim can't be
stood up, with temperature rise from 1995 to 2010
statistically significant, it may be that this new work offers
an explanation for why warming has slowed in recent years.
Entitled "Reconciling anthropogenic climate change with observed
temperature 1998-2008", the new research uses computer modelling to
assess the reasons for the slow-down, and concludes that rapid
growth in sulfur emissions from Chinese coal-fired power stations
may be offsetting some of the warming effect of rising greenhouse
gases.
Here's the short summary of the paper (the abstract), with our
emphasis:
"Given the widely noted increase in the
warming effects of rising greenhouse gas concentrations, it has
been unclear why global surface temperatures did not rise between
1998 and 2008. We find that this hiatus in warming coincides with a
period of little increase in the sum of anthropogenic and natural
forcings. Declining solar insolation as part of a normal
eleven-year cycle, and a cyclical change from an El Nino to a La
Nina dominate our measure of anthropogenic effects because
rapid growth in short-lived sulfur emissions partially
offsets rising greenhouse gas concentrations. As such, we
find that recent global temperature records are consistent with the
existing understanding of the relationship among global surface
temperature, internal variability, and radiative forcing, which
includes anthropogenic factors with well known warming and cooling
effects."
Richard Black explains the context of this diccussion for
the BBC:
"Mainstream climate scientists have
traditionally answered the "no warming since 1998" claim in two
ways. One is by pointing out that 1998 saw the strongest El Nino
conditions on record, which transfer heat from the oceans to the
atmosphere, warming the planet. So while you may not see a
temperature rise if you start the series in 1998, you do see one if
you begin with 1997 or 1999.
The second answer is to point out that temperatures will naturally
vary from year to year, and to point to the consistent upward trend
seen when long-term average temperatures are used rather than
annual figures."
Although the new paper acknowledges the that the slow-down could
be a result of natural variability in the climate system, the
authors also explored other factors which might affect temperature.
They used data on both man-made and natural drivers of global
surface temperature to simulate global surface temperature between
1999 and 2008 with a computer model.
Their conclusion:
"Results indicate that net anthropogenic
forcing rises slower than previous decades because the cooling
effects of sulfur emissions grow in tandem with the warming effects
greenhouse gas concentrations. This slow-down, along with declining
solar insolation and a change from El Nino to La Nina conditions,
enables the model to simulate the lack of warming after 1998"
So when their computer model factored in greenhouse gas
emissions, sulfur dioxide emissions and natural climate cycles, it
simulated what temperatures have done over the past 15 years.
This work suggests that rising sulfur
emissions have been offsetting the impact of rising greenhouse
gases. Sulfur dioxide is an aerosol which cools the planet by
reflecting some of the sun's energy back into space.
Burning coal is a prime cause of sulfur emissions, and as
Richard Black outlines, figures
from the US Energy Information Administration (EIA) show that
the rate at which coal is used has sharply accelerated since 2003,
particularly in China, where
electricity-generating capacity rose from just over 10
gigawatts (GW) in 2002 to over 80GW in 2006. (A large coal plant
has about 1GW capacity).
The Guardian also suggests that
"The effect also explains the lack of
global temperature rise seen
between 1940 and 1970: the effect of the sulfur emissions from
increased coal burning outpaced that of carbon emissions, until
acid rain controls were introduced, after which temperature rose
quickly."
So what does this mean for climate policy? The researchers were
fairly clear that it doesn't suggest cutting CO2 emissions is any
less important to limit climate change, with lead author Professor
Robert Kaufman telling the Guardian:
"If anything the paper suggests that
reductions in carbon emissions will be more important as China
installs scrubbers [on its coal-fired power stations], which reduce
sulfur emissions. This, and solar insolation increasing as part of
the normal solar cycle, [will mean] temperature is likely to
increase faster."