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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.

Halting emissions

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 Telegraph.

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 suddenly ceased.  

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 emissions.

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 years.

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 atmosphere.

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 temperature by 0.85 degrees.

Frolicher _zero _emissionsA 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., ( 2013)

How did the researchers interpret the results?

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 all.

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 oceans.

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.

Frölicher explains:

"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 excluded."

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 says:

"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 already spent.

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 that, here.

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 realised".

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 models".

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. DOI: 10.1038/nclimate2060

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