Climate News Network

Scientists differ on climate’s CO2 sensitivity

July 11, 2018, by Tim Radford

Coal power in Poland: Fossil fuels are just one part of a global puzzle. Image: By Petr Štefek, via Wikimedia Commons

There is another enigma in the warming puzzle: the climate’s CO2 sensitivity. How will life respond as carbon dioxide levels rise and heat soars?

LONDON, 11 July, 2018 – Scientists have yet to settle one of the biggest questions of warming: the climate’s CO2 sensitivity. How much more carbon dioxide can the atmosphere absorb – and how will life on Earth respond – before the global temperature ticks past the political milestones of 1.5°C and 2°C above the average levels for most of human history?

These were set in 2015 when 195 nations agreed in Paris to contain global warming to “well below” 2°C by 2100 and spoke openly of holding to no more than a 1.5°C average rise as the ambition.

But that means doing the sums all over again. In the last century the ratio of the greenhouse gas in the atmosphere has risen from its historic average of around 280 parts per million to more than 400 ppm. And global average temperatures have risen by around 1°C already: the world has just half a degree of leeway before the Paris target becomes impossible.

But one of the longest-running arguments in climate science is a simple uncertainty known to the professionals as “climate sensitivity.” That is: how much emitted carbon dioxide – emitted from the combustion of fossil fuels – makes a half a degree rise?

And it is a difficult question because forests, grasslands, wetlands, rivers, animals, microbes, rocks and oceans all release and absorb carbon dioxide from the atmosphere, sometimes storing it as hardwood, or peat, or carbonate rocks, sometimes releasing it as organisms decay.

“As well as being a major cause of global warming, CO2 also affects life directly”

Global temperature and carbon dioxide ratios have varied many times in pre-human history. So the human use of fossil fuels is only one component in a truly global calculation. Another factor is the area of healthy mixed forest and wetland, mangrove and prairie available to absorb that extra carbon, not to mention the algae in the warming oceans.

British scientists report in the journal Nature Climate Change that they asked the big question: how high could carbon levels get while temperatures stayed at no more than 1.5°C? They calculate that – as long as warming happens slowly – the carbon count could get as high as 765ppm. Right now, most climate researchers think that this mark will be reached or surpassed at between 425 ppm and 520 ppm.

And what makes the difference is the unresolved question of how the green things respond to all that extra carbon dioxide in the atmosphere. What difference will this make to crop yields (it is, in effect, a fertiliser), to the mix of species in the forests, and to the acidity of the oceans?

Other researchers have already asked the same question: what difference, for instance, will it make just to the tropics? And how will clouds – another factor in temperature control – respond?

Winners and losers

“As well as being a major cause of global warming, CO2 also affects life directly,” said Richard Betts, of the UK Met Office, based at the Hadley Centre in Exeter.

“Higher CO2 concentrations cause increased growth in many plant species. This causes a general ‘greening’ of vegetation, but also changes the make-up of ecosystems – some species do better than others. Slower-growing large tree species can lose out to faster-growing competitors,” he said.

“It can also reduce the effects of drought to some extent, because many plants use less water when CO2 is higher. Both of these factors can potentially enhance crop yields, possibly helping to offset some of the negative impacts of climate change – although even if that happens, the nutritional value of the crops can be reduced as a result of the extra CO2.”

But, Professor Betts warned, the same extra carbon dioxide changed the chemistry of the oceans, making sea water more acidic and potentially more damaging to corals, and to plankton.

No quick answers

The message of such research is that there are a lot more questions to be answered. Nature’s response might buy the world more time to act. But there is no guarantee.

And the Exeter reasoning has its limitations: that is because it considered only the case of carbon dioxide, and although this is the big driver of climate change, it is not the only greenhouse gas. In a warming world, the permafrost is expected to melt to release potentially colossal quantities of buried methane.

The researchers arrived at their estimates by reversing the normal reasoning. They did not try to calculate the probability of so much warming for a stipulated rise in CO2 ratios. Instead, they started with what the carbon dioxide count might look like at a particular temperature.

“This lets us estimate what the range of CO2 concentrations would be when global warming passes those levels, if CO2 were the only thing in the atmosphere that we are changing,” Professor Betts said. – Climate News Network

Print Friendly, PDF & Email
1 comment Show discussion Hide discussion
  • Charles R. Anderson, Ph.D.

    July 13, 2018 at 8:56 am Reply

    “How much more carbon dioxide can the atmosphere absorb – and how will life on Earth respond – before the global temperature ticks past the political milestones of 1.5°C and 2°C above the average levels for most of human history?” The 1.5 and 2 C milestones are not measured with respect to the average temperature for most of human history, unless you believe human history began in the 1800s when the industrial age was just getting underway in a few small areas of the world and the Little Ice Age was drawing to a close. Human history surely includes the Medieval Climate Optimum, the Roman Warming, and the Minoan Warming, each of which periods were rather favorable to the development of human civilizations.

    There is a reason why the global climate models have made wildly disparate and exaggerated predictions of the surface temperature over the last 18 years. The physics input into the models is wrong. Carbon dioxide has much less effect on the climate than is claimed by the inputs. The physics inputs violate the Conservation of Energy, fail to calculate the energy density of the electromagnetic field responsible for thermal radiation, greatly exaggerates the photon flux emitted from the Earth’s surface, hugely exaggerates the back radiation from the atmosphere, claims a degree of longwave radiation absorption by the atmosphere that a black body absorber could not absorb unless at a temperature below any temperature found in the Earth’s atmosphere, claims a false positive feedback by water vapor, and ignores the many cooling mechanisms of carbon dioxide. Garbage physics input make nonsense computer output.

Add a comment