Two new studies by NASA’s Goddard Institute for Space Studies help improve basic quantitative understanding of the Earth’s greenhouse system. The one, ‘CO2: The Thermostat that Controls Earth’s Temperature’, is a Goddard climate model based study (lead-author Andrew Lacis) to determine equilibrium temperature responses, the other, ‘Taking the Measure of the Greenhouse Effect’ (lead-author Gavin Schmidt) is a parallel study quantifying the respective total infrared absorption of all major greenhouse gases. The first was just published in Science, the second in the Journal of Geophysical Research. Combined they reach the following main conclusions:

• Water feedbacks are the main climate change engine. Water vapour contributes 50 percent to Earth’s greenhouse effect; water clouds an additional 25 percent. (Apart from reflecting solar radiation, depending on cloud type, clouds also absorb infrared radiation.)
• Non-condensing greenhouse gases contribute the remaining 25 percent of Earth’s greenhouse effect, of which CO2 is with 20 percent by far the most important, followed by methane, nitrous oxide, aerosols (soot), ozone and CFCs.
• Water influx however is itself temperature-dependant, creating an important distinction between (atmospheric, fast-acting) climate feedback greenhouse gases and climate forcing greenhouse gases, with only the latter group capable of acting as an external force on the climate system, triggering a greenhouse gas induced climate change. Therefore:
• CO2 is the most important greenhouse gas, ‘the thermostat that controls Earth’s temperature’. It is the atmospheric CO2 level that determines where a new equilibrium temperature will be reached; it is mainly the water feedback that determines how it will be reached.
• The Earth’s greenhouse system needs climate forcing gases to be sustained – 80 percent is determined by CO2.
• Without the climate forcing of non-condensing greenhouse gases (again, mainly CO2), the water feedback would turn negative and a new equilibrium temperature would form around minus 20 degrees C, on average.

Fundamental clarifying importance

Faced with the challenge of grand-scale economic reforms, in the run-up to Copenhagen, 2009, all the basics of climate science were challenged. Although most of the criticism and about all of the public ‘debate’ was utterly non-scientific, one issue did reach to the fundamentals: climate sensitivity.

We live on a green planet: CO2 + slow-acting climate feedbacks

Climate sensitivity is a term used for the expected atmospheric temperature rise for a doubling of CO2 concentrations. Combining all the relevant atmospheric research published up to the end of 2004, the IPCC in its 2007 Fourth Assessment Report (WG1, chapter 2) reached the conclusion climate sensitivity would be between 2 and 4.5 degrees Centigrade, with a 3C rise as ‘best estimate’. World leading climate researchers of NASA (James Hansen) and for instance the Potsdam Institute for Climate Impact Research (Hans Joachim Schellnhuber) have since argued true sensitivity could be twice as high when including slow climate feedbacks, like Arctic methane, deep-sea methane or increased biodegradation of ecosystems, leading to further CO2 emissions, all following an initial (industrial) CO2 induced temperature rise. These slow feedbacks lead to the runaway warming scenarios with exponential damage.

We live on The blue planet: CO2 = also fast-acting atmospheric feedbacks

The critics on the other side argue these slow feedbacks won’t be triggered as the Earth’s climate sensitivity is actually lower than IPCC AR4 states. The critics are wrong. The confusion is about the term climate sensitivity. Although it starts with a chemical greenhouse characteristic of CO2, climate sensitivity should not be interpreted as a lab result, but instead always as an integral part of the climatic context of system Earth – the blue planet, were everything depends on the abundance of water. Everything includes our climate.

Flawed focus on climate sensitivity

Somewhere over the climate politics-filled years of 2008 and 2009 the world lost track of the basics of climate science. While the new insights and publications on slow-acting climate feedbacks were worrisome to many – others hoped for comfort in denying the basic triggering factor, the climate effects of high anthropogenic CO2 emissions, mostly due to the abundant use of fossil fuels. Although the IPCC report clearly mentions fast-acting climate feedbacks, like water vapour and ice albedo, as important contributors to expected temperature rises, somehow we allowed a flawed focus to develop on the molecule of CO2 itself.

[This flawed focus exists to this day, as even BBC’s excellent green science radio show One Planet failed (podcast) to place the somewhat grumpy comments of ex IPCC [beware, the 2001 report] contributor, physicist Richard Lindzen [‘IR absorption of doubled CO2 cannot lead to 3 degrees of temperature rise’ – indeed it can’t, see the above, the difference between molecular science and climatology] in its scientific sound context. Lindzen continues to make a personal attack on the IPCC as being ‘biased’. Use your instinct. Scientists do get into rows. But even a possible bias among IPCC management can never influence the combined results of all of established climate research institutions world wide that is the IPCC.]

A giant leap backwards

All in all, with much help of controversy loving media, general damage was being done to climate science. Among much of the public the scientifically backward image that the fundamental connection between CO2 and temperature would be unproven, regained broad support. Where it is actually high time for society to focus on climate impact studies and on researching the transition to a low carbon economy, the scientific community is now forced back to re-research from the times just after Swedish chemist Svante Arrhenius reported on the warming properties of CO2, well over a century ago.

CO2 switches water feedback: climate sensitivity times four

Fortunately NASA proved willing. The two new studies now provide the important scientific background to understanding the true meaning of the concept of climate sensitivity – that should be seen as the sum of direct and the fast-acting indirect atmospheric warming effects of CO2. Firstly, all the scientific evidence points to a linear temperature rise with increasing atmospheric CO2 levels, which is in the end quite basic chemistry (the radiation absorbing characteristics of a three atom gassy molecule). This temperature rise then switches on the fast-acting climate feedback of water, primarily driven by increased evaporation and increased water capacity of the atmosphere. The new NASA study indicates this second temperature rise, although delayed from the start of increased CO2, is the more potent of the two, possibly increasing the total atmospheric temperature rise by a factor four.

Considerations on true climate sensitivity

Although NASA does not present new calculations of the actual total atmospheric climate sensitivity, they do show Earth’s climate changes are very much controlled by CO2 changes.

To offer some perspective on the range of temperature changes that a doubling of CO2 could provoke, the NASA studies do quote the difference between general CO2 stabilisation levels of ice ages and interglacials – where ecosystem changes led to a rise in atmospheric CO2 concentrations of 100 ppm (between 180 and 280 ppm), that saw an overall temperature difference of 5 degrees C. Of course the glacial episodes are primarily triggered by astronomical cycles. This however does not explain the extent and especially the duration of the cold spells (up to 100,000 years) where, according to NASA, much of the actual temperature change was created by the water feedbacks, controlled by CO2 and the other non-condensing greenhouse gases.

Present atmospheric CO2 concentrations are around 390 ppm, some 110 ppm higher than preindustrial levels. Other non-condensing greenhouse gases (also increased with industrial activity) add a little over 50 ppm in CO2 equivalents.

The NASA computations show that a world without the non-condensing greenhouse gases would see the water feedbacks turn negative and create a massive cooling – to establish a new equilibrium temperature around minus 20 degrees Centrigrade, much, much colder than any of the ice ages.

All this suggests CO2 is very important to establishing the Earth’s (delayed) equilibrium temperatures. Nothing suggests the IPCC AR4 range of 2 to 4.5 degrees climate sensitivity exaggerates reality.

Meanwhile we risk losing focus on the slow climate feedbacks. If new climate research proves the findings (‘adding slow feedbacks creates another doubling of warming’ -> 6 degrees (PDF)) of people like Hansen and Schellnhuber right, then communicators of climate science should really consider to once again extent the definition of true climate sensitivity – or establish a new term that clearly includes the (long-term) CO2-temperature responses of other Earth systems than solely the atmosphere, like oceans and terrestrial biosphere. After all, we should be interested in the net warming.

© Rolf Schuttenhelm | www.bitsofscience.org