A guest post from Colin Summerhayes, Scott Polar Research Institute, Cambridge
Charles Lyell was among the first to demonstrate, in 1830, that the world had cooled since the Cretaceous. He thought this might reflect the movement of continents across climate zones.
Alfred Wegener put the meat on the bones of Lyell’s moving continents in the 1920s, when he and Wladimir Köppen used data on past climates to position past continents in relation to latitude.
By 1970, plate tectonic theory had vindicated Wegener. We now routinely observe and model past climate change for different time slices.
Lyell knew that celestial mechanics must also play a role in controlling climate through long slow changes in the Earth’s orbit and axial tilt. In the 1860s, James Croll used astronomers’ data on the Earth’s orbit to calculate how much energy Earth received from the Sun for the past 3 million years, and for 1 million years into the future – the first climate prediction.
60 years later Milutin Milankovitch showed that northern hemisphere glaciations were driven by the amount of summer insolation at 65°N. Using computers, André Berger refined Milankovitch’s calculations in the 1970s. That enabled Nick Shackleton, Jim Hays and John Imbrie to show that variations in insolation at 65°N correlated tightly with climate change down deep sea cores – an astonishing advance in understanding how Planet Earth works.
In 1859, John Tyndall showed that H2O-vapour, CO2, N2O and O3 absorb and re-emit infrared radiation. Hence their variations in past atmospheres could explain variations in past climate. Svante Arrhenius calculated that a fall by 0.6 x modern CO2 would cool the world by 5°C, forcing a glaciation. In 1899, T.C. Chamberlin adopted Arrhenius’s findings to support a theory of climate change reliant upon changes in atmospheric CO2. But not until the 1950s would we know enough about the spectrum of CO2 in air to determine its effects on climate accurately.
Measures or estimates of past levels of atmospheric CO2 were another missing ingredient. In 1978, Hans Oeschger measured CO2 in fossil air from bubbles of ice trapped in ice cores. Now ice cores reveal records of fossil CO2 dating back 800,000 years. By the early 2000s we had also learned how to estimate past levels of CO2 from geological proxies, like the numbers of pores in fossil leaves. Over time, volcanic activity has emitted CO2, which has been absorbed from the atmosphere by chemical weathering. The balance between the two processes, controlled by plate tectonics, is what cooled our climate over the past 50 million years.
CO2 is not the only important greenhouse gas. Warming the ocean evaporates H2O as well as releasing CO2. The two act together to amplify the effects of orbital change. In the past 4000 years, orbital energy declined, leading Earth into a neoglacial period that peaked in the Little Ice Age. Subtle changes in solar output, detected by 14C and 10Be isotopes, caused minor variability about the underlying orbital trend of global cooling. Part of the reason for the exceptional cold of the Little Ice Age was a period of minimal solar output.
Where are we now? Orbitally, we are still in the Little Ice Age. Solar output is no different from what it was in the 1780s in the Little Ice Age. Since 1990 it has been declining while temperature continued rising.
Only CO2 has changed a lot, rising dramatically since about 1770, due to our emissions. The associated warming has taken temperatures above the natural climate envelope of the past 2000 years. Ice cores show that whenever temperatures went 2-3°C above today’s, in association with CO2, sea level rose by 4-9m. And when CO2 went up by more than today’s levels, the ocean acidified, dissolving carbonate sediments.
We have been warned. And this has nothing to do with fancy numerical climate models! As Bryan Lovell says – you can’t argue with a rock.
- You can find out more from ‘Earth’s Climate Evolution’, published by WILEY. For a 20% pre-production discount, use the code EES14 and contact WILEY at firstname.lastname@example.org.
- Colin Summerhayes will present this geological perspective on climate change in the GSL’s London Lectures, on April 15th. Find out more from the GSL’s ‘Statement on Climate Change’ on the GSL web site at www.geolsoc.org.uk/climaterecord.