Clearing Up the Confusion about Climate Sensitivity

In a few recent conversations with press contacts it has become clear that there is confusion regarding climate sensitivity.  More specifically, the confusion is about equilibrium climate sensitivity vs. transient climate response.  I will try to set the record straight here.

Climate sensitivity is the expected increase in equilibrium global temperature due to a doubling of pre-Industrial Revolution CO2 concentrations from 280 ppm to 560 ppm.  According to the IPCC (2007) and a number of other studies, the range of expected temperature increases due to a doubling of CO2 falls between 2C and 4.5C (3.6F and 8.1F) above pre-IR values with 3C (4.8F) the most likely.  The exact wording from IPCC is:

“Climate sensitivity is likely to be in the range 2 to 4.5C with a best estimate of about 3C, and is very unlikely to be less than 1.5C. Values substantially higher than 4.5C cannot be excluded,…”

The confusion occurs because many people believe that the moment CO2 levels reach 560 ppm the climate will be about 3C warmer.  No.  It takes decades for the climate to catch up.  The value of 3C also assumes that CO2 concentrations will remain at that constant level until the climate catches up.  Doubtful.

Transient climate response is what most people are thinking about when describing changes in global mean T due to increasing CO2.  Simply put, the transient response is the change in T at the moment CO2 levels reach 560 ppm.  According to the IPCC (2007):

It is very likely larger than 1C (1.8F) and very unlikely greater than 3C (4.8F).

The simplest way to look at this is that the planetary system (ocean, surface, air) is warming because, due to increased heat trapping from gases such as CO2, the outgoing heat to space is not as large as the incoming heat from the sun.  This means that the entire system has extra heat.  We see that extra heat going into the oceans that are warming, the air that is warming, and the ice that is melting.  To establish a thermal equilibrium (outgoing heat equals incoming heat) the entire planetary system must reach a new higher T.  Most of the net heat imbalance caused by increasing CO2 is going into the oceans.  This will continue until the surface ocean warms up enough to balance the radiative forcing.  (Thanks to Gavin Schmidt for the previous two sentences.)  For these reasons, one often hears that “there is about 0.6C (1F) warming still in the pipeline even if we stop adding CO2 today.”   See: Hansen et al. (2005) for the details.

Oceans Take Up Most of the Heat (Cook, 2010)

Unfortunately, we are increasing our CO2 emission rates thus trapping more and more heat which means the upward trend in ocean heat content, global air T, and ice melt will continue.  The climate is still playing catch-up and will do so until we level off CO2 values and start to bring them down.  Even then, there will still be warming from “the pipeline” due to the heat imbalance of previous decades.

Ocean Heat Content Increasing (NOAA, 2010)

So what temperature changes are expected down the road?  The two graphics below from the IPCC (2007) show projected changes in T following various CO2 emission paths.  Sadly, we are currently on the high emission path.

IPCC Projected T Changes for Various Emission Scenarios

IPCC Projected T Changes for Various Emission Scenarios