Evidence-based debate on climate change
Sir, In the interest of contributing to the evidence-based debate on climate change I thought it would be constructive to draw to your attention the geological evidence regarding climate change, and what it means for the future. This evidence was published in November 2010 by the Geological Society of London in a document entitled “Climate Change: Evidence from the Geological Record”, which can be found on the Society’s web page at www.geolsoc.org.uk/climaterecord
A variety of techniques is now available to document past levels of CO2 in the atmosphere, past global temperatures, past sea levels, and past levels of acidity in the ocean. What the record shows is this. The Earth’s climate has been cooling for the past 50 million years from 6-7°C above today’s global average temperatures to what we see now. That cooling led to the formation of ice caps on Antarctica 34 million years ago and in the northern hemisphere around 2.6 million years ago. The cooling was directly associated with a decline in the amount of CO2 in the atmosphere. In effect we moved from a warm “greenhouse climate” when CO2, temperature and sea level were high, and there were no ice caps, to an “icehouse climate” in which CO2, temperature and sea level are low, and there are ice caps. The driver of that change is the balance between the emission of CO2 into the atmosphere from volcanoes, and the mopping up of CO2 from the atmosphere by the weathering of rocks, especially in mountains. There was more volcanic activity in the past and there are more mountains now.
Superimposed on this broad decline in CO2 and temperature are certain events. Around 55 million years ago there was a massive additional input of carbon into the atmosphere – about four times what humans have put there. It caused temperatures to rise by a further 6°C globally and 10°C at the poles. Sea level rose by some 15 metres. Deep ocean bottom waters became acid enough to dissolve carbonate sediments and kill off calcareous bottom dwelling organisms. It took over 100,000 years for the Earth to recover from this event. More recently, during the Pliocene, around 3 million years ago, CO2 rose to levels a little higher than today’s, global temperature rose to 2-3°C above today’s level, Antarctica’s Ross Ice Shelf melted, and sea level rose by 10-25 metres.
The icehouse climate that characterised the past 2.6 million years averaged 9°C colder in the polar regions and 5°C colder globally. It was punctuated by short warm interglacial periods. We are living in one of these warm periods now – the Holocene – which started around 11,000 years ago. The glacial to interglacial variations are responses to slight changes in solar energy meeting the Earth’s surface with changes in: our planet’s orbit from circular to elliptical and back; the position of the Earth relative to the sun around the Earth’s orbit; and the tilt of the Earth’s axis. These changes recur on time scales of tens to hundreds of thousands of years. CO2 plays a key role in these changes. As the Earth begins to warm after a cold period, sea ice melts allowing CO2 to emerge from the ocean into the atmosphere. There it acts to further warm the planet through a process known as positive feedback. The same goes for another greenhouse gas, methane, which is given off from wetlands that grow as the world warms. As a result the Earth moves much more rapidly from cold to warm than it does from warm to cold. We are currently in a cooling phase of this cycle, so the Earth should be cooling slightly. Evidently it is not.
The Geological Society deduced that by adding CO2 to the atmosphere as we are now doing, we would be likely to replicate the conditions of those past times when natural emissions of CO2 warmed the world, melted ice in the polar regions, and caused sea level to rise and the oceans to become more acid. The numerical models of the climate system that are used by the meteorological community to predict the future give much the same result by considering modern climate variation alone. Thus we arrive at the same solution by two entirely independent methods. Under the circumstances the Society concluded that “emitting further large amounts of CO2 into the atmosphere over time is likely to be unwise, uncomfortable though that fact may be.”
* Vice-President Geological Society of London and Emeritus Associate Scott Polar Research Institute, Cambridge.