5.6 So how much warming is likely?
There are two aspects to this question: by how much and when? This depends crucially on ‘climate sensitivity’. And there are two important aspects to this: one is about the temperature increase due to the enhanced greenhouse effect and the other is about how long it takes for the temperature to come down from there.
The actual increase in temperature cannot be given with certainty, but involves probabilities of keeping below or exceeding various temperature limits associated with different concentrations of greenhouse gases and other emissions (including aerosols, etc.).
The table below, from the UK CCC report in December 2008, is a useful guide.
| CO2 concentration (ppm) | CO2-e concentration (ppm) | Global mean T incr above pre-industrial at equlibrium (ºC) based on climate sensitivity of 3ºC | Peaking year for CO2 emission | Change in global CO2 emissions in 2050 (% of 2000 emissions) |
|---|---|---|---|---|
| 350–400 | 445–490 | 2.0–2.4 | 2000–2015 | −85 to −50 |
| 400–440 | 490–535 | 2.4–2.8 | 2000–2020 | −60 to −30 |
| 440–485 | 535–590 | 2.8–3.2 | 2010–2030 | −30 to +5 |
| 485–570 | 590–710 | 3.2–4.0 | 2020–2060 | +10 to +60 |
| 570–660 | 710–855 | 4.0–4.9 | 2050–2080 | +25 to +85 |
| 660–791 | 855–1130 | 4.9–6.1 | 2060–2090 | +90 to +140 |
Earlier policy discussions associated avoiding dangerous climate change with 2°C warming and a stabilisation target of 550 ppm CO2-eq, but that figure has now come down to 450 ppm CO2-eq. As mentioned in the answer to ‘What is the relationship between emissions now and climate change in the future?’ in Section 5.4, 2°C warming is now widely accepted as inevitable.
However, Jim Hansen (Director of NASA's Goddard Institute for Space Studies) and colleagues argue that climate sensitivity may be as high as 6°C and that a ‘safe’ stabilisation target is 350 ppm (30 ppm less than current levels). This is based on palaeoclimatic research on the growth of the Antarctic ice cap. After a thermal maximum 50 million years ago, global temperatures cooled and atmospheric CO2 declined until it reached a critical threshold between 350–550 ppm 35 million years ago when the Antarctic ice cap started to grow. So, the reasoning is that the more atmospheric CO2 concentrations rise above 350 ppm, the more likely major environmental change becomes. Part of the reason for adopting the lower end of this range is to be clear about the direction of travel required: a marked reduction in CO2 emissions rather than giving the impression that we can get away with slowing the rate of emissions.
Although the relationship between climate sensitivity and CO2 is believed to be linear for a doubling of atmospheric CO2, it is increasingly non-linear thereafter (e.g. due to increasing positive feedbacks as terrestrial and oceanic sinks take up less CO2 from, and begin to release stored carbon to, the atmosphere).
Animation and website dedicated to the science and campaign on 350 ppm.
Briefing and discussion on ‘Target CO2’ and for climate sensitivity as high as 6°C from RealClimate.
5.5 What is climate sensitivity?