A lecture not to miss

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.

2 thoughts on “A lecture not to miss”

  1. Interesting indeed! Sadly, in the ‘may you live in interesting times’ sense…

    I note that Naish goes out of his way to point out where projections are controversial or uncertain, how large the margins of error are, and whether information is drawn from models or directly from the record, etc..

    This scarcely fits with the ‘alarmist’ caricature that we used to hear so much about here.

    That was, of course, before even the deniers began to realise that reality was not exactly running their way, and figured it might be best to slink off and, ooh, say, re-arrange the library books (you know, from Friedman to Hayek to Rand – or perhaps from ‘Right’ to ‘even further so’?) for a few months, years, or perhaps even until the Twilight Nursing Home for the Terminally Bewildered beckons*…

    And yet the dreary rump of rusted-on recalcitrants who haven’t been cluey enough to strategically evaporate still instantly ‘know for a fact’ – and beyond even the shadow of a doubt – that their shibboleth du jour is the n+1’th nail in the coffin of ‘CAGW’.

    Much as the whole peer-review process is entirely and irredeemably broken, unless, of course, one of their gaggle of heroes manages to wangle a paper across the line somewhere…

    *That’s if their youthful efforts haven’t contributed to a greater likelihood of those in their declining years ending up as Soylent Green, of course.

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