Research brief: New study suggests warming set to continue even if emissions drop to zero
- 26 Nov 2013, 11:35
- Roz Pidcock
Even if carbon emissions miraculously ground to a halt
overnight, global temperature would keep rising for centuries. At
least, that's the conclusion from a new study, which piqued the
Telegraph's interest in recent days. With the latest UN climate
report saying otherwise, we take a closer look at what emissions
cuts - fast and slow - might mean future temperatures.
In its latest climate
report, the Intergovernmental Panel on Climate Change (IPCC)
looked at what would happen if carbon dioxide emissions could be
suddenly stopped. It concluded that temperatures would decrease
only very slowly, if at all.
Now a new
paper challenges those conclusions, saying we're likely to see
temperatures continue to rise hundreds of years, even after
emissions come to a halt.
The authors say this "illustrates how difficult it may be to
reverse climate change - we stop the emissions but still get an
increase in the global mean temperature".
Where did the story come from?
The new research was carried out by a group of scientists at
Princeton University and the National Oceanic and Atmospheric
Administration (NOAA) in the United States, although lead author
Thomas Frölicher is now part of the Swiss Federal Institute of
Technology in Zurich.
Their research was published on Sunday in the journal
Nature Climate Change and covered in yesterday's
What did the research involve?
The researchers used a global climate model developed by the
Geophysical Fluid Dynamics Laboratory in Princeton to investigate
what would happen to global temperature if carbon emissions
To do this they simulated a 1,800 billion tonne pulse of carbon
into earth's atmosphere, raising the concentration to four times
pre-industrial levels. Then the researchers watched how global
temperature changed over time in the model with no further
What were the basic results?
The authors saw a rapid decrease in atmospheric carbon dioxide
for a few years after emissions stopped, followed by a slow decline
for several centuries. After 20 years, 40 per cent of the carbon
dioxide had been taken out the atmosphere by plants, land and
oceans; 60 per cent after 100 years and 80 per cent after 1,000
Temperature responded more slowly to the imposed change in
carbon dioxide emissions, peaking 15 to 20 years after they halted.
Over the next century, global temperature declined to 1.5 degrees
above pre-industrial levels as carbon dioxide dropped in the
But perhaps the most interesting result was that after about 100
years, global temperature started to increase again - by 0.37
degrees Celsius over the next four centuries.
That might not sound like much, but by way of comparison, carbon
dioxide emissions over the whole industrial era have raised global
A graph from the paper showing how global
temperature drops after emissions cease, but rises again after 100
years (red line). The blue line, which represents a global model
that does not account for a gradual decline in ocean heat uptake,
shows a slow decline in temperature over the same time.
Source: Frölicher et al., (
How did the researchers interpret the
The fact that temperatures rose in the new study after emissions
had halted goes against existing scientific research in this area,
most of which suggest global temperature would stay roughly
constant for several centuries and only decrease very slowly, if at
The authors say the difference is because most global climate
models don't account for a gradual decline in a warming oceans'
ability to absorb heat from the atmosphere, particularly the polar
With more heat staying in the atmosphere than previously
thought, this "lingering warming effect" triggers knock-on effects
to do with cloud formation. Together, these outweigh the cooling
effect of declining carbon dioxide in the atmosphere, causing
temperatures to rise.
"Scientists have thought that the
temperature stays constant or declines once emissions stop, but now
we show that the possibility of a temperature increase can not be
What are the implications?
The last IPCC report included an estimate of how much carbon we
can emit and still have a reasonable chance of staying below two
degrees - the widely accepted political threshold beyond which the
risks posed by climate change are considered unacceptably high.
According to the IPCC's calculations, this
budget is equivalent to 1000 billion tonnes of carbon. But the
new research implies this may be overly optimistic. Frölicher
"If our results are correct, the total
carbon emissions required to stay below two degrees of warming
would have to be three quarters of previous estimates, only 750
billion tonnes instead of 1,000 billion tonnes of carbon."
Since humans have already contributed 515 billion tonnes of
carbon to the atmosphere over the industrial era, that leaves less
than 250 billion tonnes of the budget remaining, say the authors.
In other words, just half the carbon emissions we've
In fact, the IPCC says the two degrees budget is likely to be
tighter than 1000 billion tonnes once you take into account other
non-carbon dioxide factors that cause warming, such as methane,
CFCs, ozone, nitrous oxide and black carbon. We wrote more about
Another implication of the new research is that
some estimates of how sensitive the climate is to a given
amount of carbon dioxide may be too low if they assume heat
transfer between the atmosphere and oceans stays constant over
time. The authors say in the paper:
"If the real world were to behave in a
similar manner as [our model] then only a fraction of the total
warming due to past carbon emissions has thus far been
But Jerry Meehl from the US National Centre for Atmospheric
Research, who has carried out similar research, warns of attaching
too much importance to one result. He tells us:
"This result is from one model, and
depends on that model having a pattern of ocean heat uptake
that triggers cloud feedbacks that would provide
slight increases of global temperature for a while, rather than not
much temperature change or slight decreases. Since the authors
don't say much about the nature of these cloud feedbacks in this
model, it can only point to further research to identify
those processes and how they could extend to other
What else should we know?
The new research talks about an idealised situation in which
emissions are cut to zero effectively overnight. That's not likely
to happen in the real world.
A more realistic outcome might be to maintain emissions at
current levels. The IPCC suggests we'd see temperatures rise by
a further one to 2.5 degrees Celsius by 2100, depending on the
models' estimates of climate sensitivity.
Even with a rapid decarbonisation effort, we'd be committed to a
certain degree of warming over the time it takes to undergo a
transition of the energy system, the report adds.
While the IPCC does not comment on the social, political and
economic factors controlling the speed of such a transition, a
study released last week
highlights how each year we delay cutting emissions puts
staying below a given threshold a little further out of reach.
The new study will need to be rigorously tested before its
conclusions are considered reliable, but it could suggest the
chances of staying below two degrees is even slimmer.
Source: Frölicher et al. (2013) Continued global
warming after CO2 emissions stoppage. Nature Climate Change.