Much has changed since February 2004 when the Observer published this rather sensational tabloid-beating headline. For one, the USA, still the largest economy in the world with the greatest fossil fuel emissions, has a President who actually believes in global warming. Climate change predictions have been updated through several reports detailing the current climate situation and the likelihood of abrupt climate warming (IPCC 2007, USGS 2008 and the upcoming IPCC 5th Report in 2014) and the world has started to attempt to actually do something about global warming through COP15, the EU ETS and other similar agreements rather than simply accepting such sensational headlines as fact.
However, there is still the issue of how likely an abrupt climate change event could take place over the next century. Parts of this blog have discussed changes which have occurred over periods less than human life expectancies including the breakdown of the Younger Dryas. Therefore, full understanding of abrupt climate change likelihood in the future is particularly important.
First let's look at the USGS 2008 report on abrupt climate change. The report is summarised into 4 sections to assess whether there will be an abrupt sea level rise, a change in the hydrological cycle, a weakening of the AMOC or an abrupt change in atmospheric methane.
Taking the AMOC first, since this blog has reported its weakening as a major cause of abrupt climate change events in the past, the USGS suggests that the AMOC will decrease by between 25-30%. This will be caused by both natural variability and anthropogenic warming. As a result, there will be less heat transfer to the North Atlantic from the thermohaline circulation. Despite this, it is expected that a warming trend will still occur over Europe and North America as temperatures from global warming increase. At present, there are no models which suggest that a complete collapse of the AMOC will occur within this century. However, if the AMOC were to collapse, an 80cm sea level rise in the North Atlantic and a net cooling of 1-2C on top of increased temperatures in response to greenhouse gas forcing. Vellinga and Wood (2008) (2) suggest that temperatures could decrease by as much as 8C locally and that, once the climate had stabilised, the AMOC would take 100 years to recover.
Secondly, the USGS report suggests that an abrupt sea level rise caused by the melting of ice caps is "possible". Evidence has been presented showing that the West Antarctic and Greenland ice sheets are both thinning and breaking off which would cause a sea level rise much larger than predicted by the IPCC 4th Assessment Report. Hydrological changes causing water scarcity and drought in many sub-tropical areas are extremely possible. Global water extraction is set to increase by 31% by 2020 as predicted by the OECD (3) in 2003 from 1995 levels. Indeed, my university dissertation which modelled the climate change effects on the volume of discharge in Rio Grande and Arkansas rivers, two of the largest rivers in the USA supporting millions of people, found a possible 5% decrease in discharge volume by 2050 despite population size doubling (Shefford, 2011) (4).
The threat of a methane concentration rise is also real. Methane hydrates, which lie trapped in permafrost and in the deep oceans, may be released to the atmosphere causing further global warming. It appears to be very unlikely that such changes will occur in the near future but changes within 1,000-100,000 years may well be likely.
The USGS report has suggested that changes to the AMOC are unlikely along with the release of methane hydrates. However, the possibility of a northward movement of drought zones and sea level rise could well give some communities severe issues in the 21st century.
Now there are always the sensationalists out there but this paper by Schwartz and Randall (2003) (5) pushes the boundaries. They suggest exercising caution in the likelihood of their scenarios but they have pushed their models over a temperature threshold so that the worst possible event occurs. When temperatures rise above a threshold, abrupt changes will occur causing a 5-10 F temperature change in just a decade. They suggest that these consequences could last 100 years and be analogous to the 8200-year event or even the Younger Dryas. This blog has taught me to be critical and unless they can find an invisible Lake Agassiz from which freshwater outbursts to the North Atlantic can occur, I see no support of a future Younger Dryas or 8200-year event. Yes the North Atlantic has freshened, as reported by Dickson et al (2002) (6), but I can't imagine where the freshwater forcing as seen in the Younger Dryas or 8200-year event would come from to cause the collapse of the AMOC.
So was the Pentagon correct? In some ways yes, drought will likely cripple parts of the USA with sea level rise also set to affect its low-lying cities. If we're talking about the world over, the AMOC should survive anthropogenic warming at least until the 21st century ends. However, maybe after the USA's performance after Kyoto and COP15, "us" really does mean "U.S."!
In my final post, I will conclude this blog by summarising what my research has found and where this leaves us for the near future of abrupt climate change events.
(1) Delworth et al., (2008) USGS-CCSP 3.4
(2) Vellinga and Wood (2008) doi: 10.1007/s10584-006-9146-y
(3) OECD (2003) Improving Water Management: Recent OECD Experience
(4) Shefford (2011) unpublished.
(5) Schwartz and Randall (2003) Environmental Defense Fund
(6) Dickson et al (2002) doi: 10.1038/416832a