Climate science in a nutshell – Part 1
James Hansen’s Storms of my Grandchildren has been affectionately described as having “a rambling quality that’s sometimes evident in Hansen’s speeches”. However, the same reviewer also noted that “the book does get to a focused and well-supported conclusion: that business as usual will bring climate catastrophe, and our time to act is running short”.
Based on over 20 years of professional hydrogeological work experience, I believe that probabilistic computer models are useful, as does James Hansen. Undoubtedly, he would be very quick to point out, as does Clive Hamilton, that the only uncertainty in model predictions arises out of uncertainty in emissions forecasts; and not out of any uncertainty regarding climate sensitivity to changes in atmospheric CO2. Despite all this, the most remarkable and challenging thing about Hansen’s book is its focus on what we can learn from studying changes to the Earth’s climate over geological timescales (i.e. palaeoclimatology). It is therefore clear that Hansen sees this, rather than climate modelling, as the best available evidence of the need for urgent recognition of – and response to – the fact that human activity is endangering all life on Earth.
In pages 36 to 51 of Storms of my Grandchildren, Hansen provides an introduction to what we can learn from palaeoclimatology. However, one basic fact is not clearly stated, which is a shame. This fact, however, is clearly stated in the first episode of David Attenborough’s new BBC series, Frozen Planet: Most people are probably familiar with the fact that warm air holds more moisture than cold air, but how many are aware that water at 30 Celsius can only hold half as much dissolved CO2 as it can at 10 Celsius? Furthermore, how many could say why this is so fundamentally important to climate science?
In this post and those that follow it this week, I intend to unpack all this sort of stuff; and demonstrate why the time is well overdue for climate sceptics to stop denying that anthropogenic global warming (AGW) is happening, significant and bad; and why it is therefore a problem that we simply must try and mitigate. However, for now, let us just focus on this issue of CO2 solubility in water, which is a key to understanding the palaeoclimatic record (e.g. as revealed from studying ice core data from holes drilled through the Antarctic ice cap).
Although Antarctica was first glaciated (i.e. covered in ice) 35 million years ago, the current ice cap (over 4 kilometres thick in places) only provides a record of several 100 thousand years but, believe me, this is enough for us to understand how planet Earth works. From this data, we know that there have been 8 ice ages in the last 750,000 years, and we know that they were all caused by regular wobbles in the angle of the Earth’s axis of rotation and less-regular variations in the non-circularity (i.e. eccentricity) of its orbit. We also know that changes in the CO2 content of the atmosphere, which match those in the Earth’s temperature exactly, happened several hundred years later in each and every case. This is because of the way in which the Earth regulates its temperature (ensuring an energy balance between incoming solar radiation and outgoing infra-red radiation) using the solubility of CO2 in water:
Put very simply, when the oceans cool down (during the onset of an ice age), they absorb CO2 from the atmosphere and, because it has been removed from the atmosphere, more heat is lost to space. Thus the Earth stays cold until the natural forcing induces warming (e.g. axis of rotation becomes less vertical again). Then, as the oceans warm up they release CO2 into the atmosphere, trapping more heat in, keeping the planet warm. Thus, both glacial and interglacial periods represent times of energy balance; whereas periods of transition represent times of energy imbalance that are eliminated by atmospheric CO2 changes.
The next step in understanding palaeoclimatic changes is to differentiate between forcings (that bring about change) and feedbacks (that amplify or accelerate any change). It is the existence of amplifying feedback mechanisms that make artificial changes to the atmospheric CO2 concentration so dangerous. However, quite how dangerous, we can only discover by looking in detail at changes that have occurred over the last 400,000 years. This, then, is what I will do tomorrow.