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Monday, July 13, 2009

Tim Naish: Evidence for anthropogenic climate change is unassailable

Unassailable Evidence

Tim Naish

Tim Naish

Inaugural Lectures are very traditional university events, held these days to permit outstanding researchers to present an overview of their work to the wider academic community and public. Last night Prof. Tim Naish delivered a clear and decisive verdict about human-induced climate change: the evidence is unassailable.

Held in the very formal surrounds of the Hunter Building Council Chambers, the room was full to what I would guess its capacity of 400-500 people. His presentation wove together threads of the science, his own career, acknowlegements to those who went before him and the numerous mentors and colleagues whom all modern researchers depend on. Modern science is a very complex business; while the rare lone genius occasionally contributes ground-breaking insights, the vast bulk of progress these days is made by teams of people supporting each other, and all standing on the shoulders of those who went before them.

Starting with the Scottish mathemetician James Croll (1870), who was the first to deduce and calculate what are now more popularly known as the Milankovitch (1930s) cycles, Naish gave a brief introduction to the naturally occurring Ice Ages and sea-level changes that accompany them. The next major figure referenced was Sir Nicholas Shackleton (1937-2006), a pioneer in the use of mass spectrometry to determine changes in climate as recorded in the oxygen isotope composition of calcareous microfossils. Using ocean sediment cores, his team convincingly demonstrated that oscillations in climate over the past few million years could be correlated with variations in the orbital and positional relationship between the Earth and the Sun. However, even late in his life, Shackleton realised that the underpinning ideas of his work were only theoretical, that corroborating physical evidence was still required.

Tim Naish’s PhD work (1980s) studying sedimentary sequences in the Wanganui basin provided that evidence. The Wangaui basin is a 5-km-deep depression in the earth’s crust that has been filled over millions of years with erosion material from the continuously uplifting Southern Alps, material that is driven north up the West Coast by part of the Great Oceanic conveyor currents to form layer upon layer of beach material in the basin. Within that record is embedded clear evidence of numerous, dramatic sea-level changes, often in the order of 50-100 m, that exactly correlate with Shackleton’s records. Subsequent work has detailed a precise and robust sequence of almost 100 such oscillations, going back millions of years.

After working for Bob Carter for a number of years, Naish joined GNS (Geologic and Nuclear Sciences CRI based in Seaview Lower Hutt and closely attached to Victoria University), initially as a petroleum geologist. Very quickly he became involved in various Antarctic drilling programs, the first at Cape Roberts. This last summer, he was the Science Team leader for the ANDRILL project, an ambitious and successful core drill sited on the Ross Sea Ice shelf. Essentially, the drill penetrated 80 m of floating ice, 800 m to the sea bed, and then another 1000 m into the sedimetary layers beneath. (This alone was a remarkable technical tour-de-force and currently a world record.)

Most such core drills only recover about 40% of the potential record, but a specialist New Zealand company has pioneered innovative techniques allowing them to recover 98% of the material, greatly enhancing the quality of the data that can be derived from them. In the 2007-2008 season, the team’s sedimentologists identified 60 cycles when ice sheets or glaciers advanced and retreated across McMurdo Sound, dating back at least 5 million years.

Now for the takeaway message.

Over the last 3 million years, the Earth’s climate has undergone at least 60 naturally driven Ice Age cycles, each accompanied by major changes in sea level. As the temperature changes, so does the CO2 level. Over very long time scales, it is apparent that the Earth is gradually cooling, but the amplitude of the cycle changes is of the same order, i.e., the peak warm periods are almost as warm as they were million years ago, before gradually sinking back into the next Ice Age. Critically, it is vital to understand that the warm peaks of this natural glacial cycle always coincide with a peak CO2 level of not more than 300 ppm. [Blogger's note: we are pretty much at 390 ppm, now.]

Normally, these changes take place over many tens of thousands of years. Humans, however, have taken CO2 from about 280 ppm pre-Industrial to almost 400 ppm in around 100 years. In geological/climatic terms, this time scale is less than an eye-blink; it is a massive, virtually instantaneous, shock to the system. The last time the Earth’s climate had this much CO2 in it was about 3 million years ago when the climate was 5-6 °C warmer, there was no ice at either pole, and sea level was about 100 m higher than present. The major reason why this has not already happened is that the oceans represent a monstrous thermal mass, and the time lag for them to respond will be in the order of many hundreds of years, but respond they inevitably will. And it is the warming oceans that melt the Antarctic Ice shelves.

The ANDRILL sediments clearly show that during the warm periods the Antarctic oceans were full of algae blooms and the sea temperature was at least 5 °C. No ice existed in Antarctica during these warm inter-glacial periods. None at all.

There are two Antarctic ice shelves. The Eastern Ice shelf (EAIS) is by far the larger, grounded mostly above sea level and considered relatively stable for the foreseeable future. The West Antarctic Ice Shelf (WAIS) is totally different. Much of it is grounded at depths of 1-2 km below sea level. Therefore, it is peculiarly exposed to the effects of warming oceans in a potentially unstable and difficult to predict fashion.

The current IPCC’s mainstream predictions are of a 0.5 m sea-level rise by 2100, and exclude melting of the Greenland ice sheet and WAIS because at the time of publishing the IPCC considered the science around them too uncertain. The ANDRILL work has greatly reduced one aspect of that uncertainty. Alone, the linearly projected melting of the WAIS will add another 0.5 m of sea-level rise to the IPCC figure. A total of 1-m rise by 2100 is now considered a mainstream prediction in the community.

Worse still, Naish and his colleagues are now faced with clear evidence that the WAIS does not necessarily melt in a linear fashion, rather it is prone to highly unstable events that could lead to massive breakups, potentially adding up to 3.2 m of average sea-level rise in quite short periods of time. And, due to the way the earth’s gravitational field works, that average rise would not be distributed evenly over the earth’s surface; in some places like North America the rise could be up to 4.0 m… within our, or our children’s, lifetimes.

In the longer run, CO2 over 400 ppm commits the climate to a complete loss of the WAIS, the Greenland ice sheet and EAIS, totalling a sea-level rise of about 100 m. The evidence is now unassailable; a firm commitment to Copenhagen later this year is our last chance to act. Failure will bring only our grandchildren’s condemnation.

(And yes I took the train into town.)

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