A lecture not to miss

By Bryan Walker 17/09/2011

Tim Naish’s lecture, of which we gave notice recently, is now recorded on the Climate Change Research Institute’s website. I warmly recommend it for viewing. Naish is one of the lead authors for the paleoclimate chapter for working group 1 of the IPCC Fifth Assessment Report due in 2013. In this lecture he uses paleoclimate material to provide perspective for the projections of rising global temperature and climate change. We are headed for climates and temperatures that haven’t been seen on the planet for more than a million years and the paleoclimate record helps us to understand what we might expect in terms of polar ice behaviour and sea level rise.

In fact we have to go back 3 million years — to the mid-Pliocene —  before we see temperatures like those the models are projecting, 2 to 3 degrees warmer by 2100. The atmospheric CO2 level then was about 400 parts per million. This Pliocene warm period is becoming an important window into what we might expect incoming decades.

The lecture addresses the importance of polar amplification of global warming, its impact on the ice sheets, and their impact on sea level rise.  It covers a range of questions, including the difficulty of dealing with any non-linear response of ice sheets, and the difference between West Antarctic ice sheet melting where warmer seas will affect the parts of the ice sheet which are below sea level and Greenland where the melt-down is from above and where it is difficult to estimate the likely consequences. The jury is really out on Greenland. We are committed to a certain amount of loss from West Antarctica because it’s the ocean that is responsible. We could potentially still save Greenland because it’s melting from the top down. But at the moment both are contributing at equal rates to sea level rise.

Observations of sea level rise are consistently higher than the IPCC projections. Naish explains Rahmstorf’s recent work on the semi-empirical relationship between temperature rise and sea level rise, which without considering any non-linear ice sheet dynamic estimates 2-3 degrees of warming will lead to about a metre of sea level rise. He produces a map showing the uneven distribution of the rise, remarking that New Zealand is likely to get more than a metre and the Pacific Islands more again, with disastrous results for them.

He notes surprise that when the world was 2-3 degrees warmer 3 million years ago there was a 20 metre rise in sea level. Even more surprising is the last interglacial period 125,000 years ago: although CO2 concentration was at pre-industrial levels and temperatures were not much higher than today sea level rose 6 to 9 metres.

The last part of the lecture deals with the expected rate of sea level rise.

Here is the slide summary of his conclusions:

  • Paleoclimate models and geological records provide important constraints on ice sheet response and sea-level rise for global temperatures projected for coming centuries.
  • Best estimate from models and observations is ~+1m±0.5 by 2100.
  • Geological data and models suggest a ’most likely’ rate of 1m per century
  • The detailed trajectory of sea-level rise of the coming centuries will be controlled by non-linear ice sheet dynamics —a major modelling challenge
  • The last time Earth had ~400ppm CO2atmosphere, it had an average surface temperature of 2-3C higher than today and Greenland and West Antarctic Ice Sheets melted!

This just a sample from the lecture, intended to encourage listening to it. It is well worth 45 minutes of time. Naish is admirably clear and accessible, and his material is presented in the context of developing scientific understanding; there’s full recognition of uncertainties but no doubting the direction in which the science is pointing.

Here’s the recording, and here’s the pdf of the slides. Be warned that the first minute of the recording — the introduction — is very poor quality sound, but thereafter it’s fine.