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Climate Change Statement Addendum

The Geological Society has published an addendum to Climate Change: Evidence from the Geological Record (November 2010) taking account of new research, writes Nic Bilham

The reconvened working group and Council concluded that the 2010 Statement remained valid, and did not need amending. The addendum sets out new research relevant to the questions addressed in the original statement.

Below follows the non-technical summary of key points from the addendum. The full technical version with references may be read online. It is intended to be read alongside the original 2010 Statement, and follows the same Q&A format.

SUMMARY

“Since our original 2010 statement, new climate data from the geological record have arisen which strengthen the statement’s original conclusion that CO2 is a major modifier of the climate system, and that human activities are responsible for recent warming.

“Palaeoclimate records are now being used widely to test the validity of computer climate models used to predict climate change. Palaeoclimate models can simulate the large-scale gradients of past change, but tend not to accurately reproduce fine-scale spatial patterns. They also have a tendency to underestimate the magnitude of past changes. Nevertheless they are proving to be increasingly useful tools to aid thinking about the nature and extent of past change, by providing a global picture where palaeoclimate data are geographically limited.

“Geologists have recently contributed to improved estimates of climate sensitivity (defined as the increase in global mean temperature resulting from a doubling in atmospheric CO2 levels). Studies of the Last Glacial Maximum (about 20,000 years ago) suggest that the climate sensitivity, based on rapidly acting factors like snow melt, ice melt and the behaviour of clouds and water vapour, lies in the range 1.5°C to 6.4°C. Recent research has given rise to the concept of ‘Earth System sensitivity’, which also takes account of slow acting factors like the decay of large ice sheets and the operation of the full carbon cycle, to estimate the full sensitivity of the Earth System to a doubling of CO2. It is estimated that this could be double the climate sensitivity.

“When viewed in the context of geological time, today’s conditions are atypical. We are living through an interglacial period, whose mean temperature is representative of only 10% of the last 800,000 years. The other 90% of that time, temperatures were lower, ice sheets larger and sea levels lower. This highlights how unusual current temperatures, and estimated future warming, are.

“Before the current warming trend began, temperatures in the Holocene (the last 11,000 years) were declining. This was due largely to insolation – the solar radiation received by the Earth’s surface – and dictated by the Earth’s orbit and the tilt of the Earth’s axis. Insolation declined throughout the Holocene. This cooling took Earth’s climate into a Neoglacial period, culminating in the ‘Little Ice Age’ (1450 – 1850).

“Astronomical calculations indicate that this period of low insolation and associated cool conditions should continue for about another 1000 years. Nevertheless, after 1900 the overall decline in temperature sharply reversed. According to one recent study, it is likely that the area-weighted global average temperature for the 30 year period from 1970 to 2000 was higher than at any time in nearly 1400 years. Tree ring data confirm that recent warming is unprecedented in central Europe over the past 2500 years, and in eastern Europe over the past 1000 years. Palaeoclimate records from the Arctic show that the warmest 50-year interval of the past 2000 years occurred between 1950 and 2000 AD.

“Atmospheric CO2 is currently just below 400 parts per million (ppm) on average. It last reached similar levels during the Pliocene (5.3-2.6 million years ago). At that time, temperatures rose to levels 2-3°C warmer than today, and sea level rose by up to 20m in places. Sea level takes a few hundred years to reach equilibrium in response to changes in atmospheric CO2 and temperature, which may explain why sea level has not yet risen to the same levels seen in the Pliocene.

“Atmospheric CO2 is increasing at around 2ppm per year (1995-2010 average). If this rate continues, it may reach 600ppm by the end of this century – a value that appears not to have been typical for at least 24 million years.

“Our 2010 statement suggested that the rise in Antarctic temperature at the end of the Last Glacial Maximum (approximately 20,000 years ago) began a few centuries before CO2 showed any reaction. New data now indicate that CO2 rose at the same time as Antarctic temperature, and ahead of the global rise in temperature. This strengthens the argument that rises in CO2 levels triggered by regional factors were instrumental in triggering global temperature increases, with positive climate feedbacks magnifying this effect.

“There is now greater confidence that a relatively modest rise in atmospheric CO2 levels and temperatures results in significant (though not globally uniform) sea level rise. Increased CO2 in the atmosphere also increases CO2 levels in the oceans, making sea water slightly more acidic and less oxygenated. In past warming events, such as at the Paleocene-Eocene Thermal Maximum (PETM) 55 million years ago, this caused marine crises and extinctions. The Earth System usually takes around 100,000 years to recover from such events.

“Given the above, based on a growing abundance of palaeoclimate data, there is now greater confidence than in 2010 that the only plausible explanation for the rate and extent of temperature increase since 1900 is the exponential rise in CO2 and other greenhouse gases in the atmosphere since the Industrial Revolution. This rate of increase of CO2 is unprecedented, even in comparison with the massive injection of carbon into the atmosphere 55 million years ago that led to the major PETM warming event, and is likely to lead to a similar rise in both temperature and sea level.”

  • Read the full Addendum at www.geolsoc.org.uk/climatechange