Should we bet the future of the world’s economy based on AR5 projections of future climate change?
The earth’s climate has been dominated by a series of glaciations for the last 3 million years. For the last 800,000 years brief warm interglacial periods have coincided with maximum eccentricity of the Earth’s orbit on a 100,000 year cycle. These swings in climate are far more intense than any recent global warming – figure 1.
IPCC scientists remain confident that their models accurately explain global warming. We read in D.1 in the SPM : “Observational and model studies of temperature change, climate feedbacks and changes in the Earth’s energy budget together provide confidence in the magnitude of global warming in response to past and future forcing.”
So I have a simple challenge for the modelers to prove this claim. If they are so confident in their models then they should also be able to accurately reproduce the dynamical changes in climate that occurred during the last 100,000 year glacial cycle in response to orbital forcing. The Milankovitz cycles are known exactly, as are past CO2 levels extracted from ice cores. Therefore it should be straightforward to run the models over the past 120,000 years and reproduce the trends observed over the last ice age period. It is in fact the ultimate hindcast.
Modern GCMs include “sophisticated” models of the cryosphere including ice melting and albedo feedbacks. They also model the atmosphere/ocean circulation, water and carbon cycles and associated feedbacks. These same models then “project” future anthropogenic global warming. So exactly just how well can they also reproduce orbital forcing of ice ages ?
Currently models normally are run an a time interval of one day which follow the seasonal insolation based on current Earth’s orbital parameters. Projections for AGW are typically run for periods up to 100 years based on CO2 emission scenarios and other anthropogenic effects.
However modeling past climates is different. To simulate glaciations model runs over 500,000 years including Milankovitz forcings are needed. This could probably be done by making a yearly run once every 1000 year interval and adjusting the orbital parameters acordingly. I wonder if this has this ever been tried ? As far as I know the models are unable to predict the dynamics of the glacial cycle, although static runs have been made at the last glacial maximum (LGM) and correctly explained energy balance, but that is not the same as describing the dynamics of glaciation.
So in my opinion we should remain sceptical about future warming projections until they can also reproduce past glacial glacial cycles. Despite this AR5 authors state essentially that without human CO2 rises they would be able to predict the next glaciation as follows.
5.8.3 Next Glacial Inception
Since orbital forcing can be accurately calculated for the future (see Section 5.2.1), efforts can be made to predict the onset of the next glacial period. However, the glaciation threshold depends not only on insolation but also on the atmospheric CO2 concentration (Archer and Ganopolski, 2005). Models of different complexity have been used to investigate the response to orbital forcing in the future for a range of atmospheric CO2 levels. These results consistently show that a glacial inception is not expected to happen within the next approximate 50 kyr if either atmospheric CO2 concentration remains above 300 ppm or cumulative carbon emissions exceed 1000 PgC (Loutre and Berger, 2000; Archer and Ganopolski, 2005; Cochelin et al., 2006). Only if atmospheric CO2 content was below the pre-industrial level would a glaciation be possible under present orbital configuration (Loutre and Berger, 2000; Cochelin et al., 2006; Kutzbach et al., 2011; Vettoretti and Peltier, 2011; Tzedakis et al., 2012a). Simulations with climate-carbon cycle models show multi-millennial life-time of the anthropogenic CO2 in the atmosphere (see Box 6.1). Even for the lowest RCP 2.6 scenario, atmospheric CO2 concentrations will exceed 300 ppm until the year 3000. It is therefore virtually certain that orbital forcing will not trigger a glacial inception before the end of the next millennium.
So why don’t they simply explain the previous glaciation starting 100,000 years ago when there were no CO2 emissions? There are accurate data regarding the changes in insolation in magnitude and with latitude. My calculations of changes in net polar insolation are shown in figure 2.
I previously analyzed the Ice Age cycles by fitting them to a combination of the precession signal, the obliqueness signal and the elipticity signal. These results shown in figure 3 allow an extrapolation forward in time to predict the next ice age.
The current glacial cycle has similar orbital parameters to the one occuring 5 glaciations ago. That interglacial warm period lasted 10,000 years or about the length of the current one. We are due for another ice age.
We forget how short human history really is. Homo Sapiens evolved in the African Rift valley only 200,000 years ago during the previous glaciation. Ice ages have driven human evolution and the consequent migration out of Africa. At the last glacial maximum (LGM) most of northern Europe and North America lay under Ice glaciers. During the last glaciation the Sahara desert was grasslands and humans migrated north first to the middle east and then along the coast to Asia eventually reaching Europe, Australia and North America. Essentially humanity spread across the world during the last glaciation in a race for survival. Civilization only developed during the last 10,000 years warm inter-glacial period. Life flourishes in warm climates rather than cold climates !
If climate models eventually succeed to hindcast past glacial cycles then I will willingly accept their ability to project anthropogenic global warming and its impact on future glacial cycles. Perhaps by then global warming will turn out to be our savior and we will need to keep CO2 levels above 400ppm just to avoid another devastating glaciation.