Arguments
The Last Interglacial Part Five - A Crystal Ball?
Posted on 17 November 2011 by Steve Brown, jg
Note: This is the fifth and final article of a five-part series on what we can learn from the Last Interglacial time period. Understanding this period may provide clues on how the environment may respond to similar conditions in the future. In the first post, we described the conditions that exisited during the Last Interglacial. In the second post, we looked at the key factors for making it a warm period. In the third post, we looked at how sea-levels rose as a result of melting ice-sheets. In the fourth post, we examined how the Last Interglacial oceans influenced the climate. In this final post, we summarise what we've learnt and conclude the discussion on the value of the Last Interglacial for helping us predict the future.
Comparing the influences on the Last Interglacial climate with those assumed in future climate projections is problematic owing to fundamental differences, especially orbital forcing, seasonal warming, and greenhouse gas concentrations. Palaeoclimate studies show that differences in the manner in which the Earth orbited the Sun during the Last Interglacial are sufficient to explain the higher temperatures over most parts of the Northern Hemisphere, particularly due to greater axial tilt and eccentricity compared with the present day orbital configuration. This greater axial tilt provided stronger insolation (solar heating) at high latitudes and weaker insolation at low latitudes. Perihelion, when the Earth is nearest the Sun, occurred during summer in the Northern Hemisphere, which amplified seasonal insolation, while perihelion occurs in winter during the present day. Figure 1 shows a comparison of Eemian and Holocene summer insolation, which highlights the key differences between then and now. If you click on Figure 1, it will show an animation to make this easier to visualise. Peak insolation from orbital forcing will be significantly lower over the next century than what the Earth received during the peak warm period around 126,000 years ago.
Figure 1: Comparison of Eemian and Holocene Summer Insolation - click for animation (Illustration and animation by jg)
The Last Interglacial climate was characterised by strong seasonality with warmer summers and cooler winters, which does not compare with the outcomes of predicted future greenhouse warming where both warmer winters and summers are expected. Warmer winters may encourage the poleward spread of coniferous forests that would accelerate snow and ice melting during the Arctic summer.
Atmospheric CO2 varied between 250 to 300 ppmv. Atmospheric CO2 during the Last Interglacial was comparable to the pre-industrial Holocene and reasonably stable, which prevents the period being a good analogue for future climate. Today, atmospheric CO2 is much higher; ~390 ppm and increasing by ~2 ppm yr-1, giving a higher contribution to radiative forcing than would be expected during the Last Interglacial.
Predictions of future sea-level rise and reduction in volume of ice sheets are consistent with what the evidence indicates during the Last Interglacial. The IPCC AR4 report suggests a complete disappearance of the Greenland Ice Sheet during the Last Interglacial, providing a contribution of up to 2 to 4 m of sea-level rise, while Cuffey and Marshall (2000) estimate that the Greenland Ice Sheet melt contributed towards 4 to 5.5 m of sea-level rise. According to Allison et al. (2009), as much as 2 m of sea-level rise could happen by 2100 with predicted global mean warming of +2.0oC. However, it should be recognised that the equivalent warming during the Last Interglacial occurred over thousands of years and the thermal response of ice and oceans would be much slower. Field et al. (1994) and Seidenkrantz & Knudsen (1997) raise the possibility that predicted higher sea-level could cause warming of the North Sea and cause warmer temperatures in northern Europe due to warm water from the North Atlantic Drift to pass through the English Channel into the North Sea and opening sea-ways between the North and Baltic Seas. A stronger North Atlantic Drift might also increase the flow of warm water into the North Sea past northern Britain. However, this may be mitigated if the present isostatic rebound of the northwest European crust outpaces greenhouse warming induced sea-level rise (Burman & Passe, 2008).
The dynamics of the Atlantic Meridional Overturning Circulation (AMOC) and its influence on climate is hotly debated and evidence for its behaviour during the Last Interglacial may have potential in determining how ocean circulation and related climatic effects may alter in a future warming world. Most models that investigate increasing greenhouse gas scenarios predict that the AMOC will slow down as a result of such forcing (Driesschaert et al., 2007; Hodell et al., 2009). However, other model results suggest that anthropogenic aerosols may have delayed a greenhouse gas induced weakening of the AMOC by reflecting inbound solar radiation and partially offsetting greenhouse gas warming (Vellinga et al., 2008). Crowley and Kim (1992) and Lisiecki et al. (2008) show that the AMOC may be sensitive to different orbital parameters, such as maxima in obliquity and precession, and link orbital eccentricity to North Atlantic Deep Water production.
Some climate models of the Last Interglacial suggest that a high-index state of the North Atlantic Oscillation (NAO) was favoured due to the configuration of orbital parameters. Less vigorous ocean circulation in the North Atlantic has been associated with positive phases of NAO. If peak summer insolation during the early Last Interglacial favoured a persistent positive NAO, this may have contributed to the slow circulation from 128 to 124 ka. (Hodell et al., 2009), as well as AMOC anomalies due to heat flux and surface wind stress variability (Vellinga et al., 2008). With a significantly different orbital configuration over the coming centuries it cannot be expected that the behaviour of the NAO and AMOC would be similar to that during the Last Interglacial.
Comparison of the behaviour of the El Nino Southern Oscillation (ENSO) during the Last Interglacial and the recent past indicate that it may be changing in response to increased greenhouse gasses. Hughen et al. (1999) suggests a robust ENSO variability that matches ENSO behaviour in the recent past. The similarity appears to diverge from observations of ENSO variability since the 1970's indicating that ENSO since then is anomalous to natural variability i.e. ENSO may be relatively stable during interglacials. El Nino events have become more frequent since the 1970's compared with the warmer Last Interglacial, which prompts the hypothesis that the higher frequency may be related to the rate of greenhouse gas induced warming rather than warmer mean conditions.
Figure 2: Comparison of the Eemian and the Holocene Interglacials (Illustration by jg)
The difference in greenhouse gas concentration combined with significantly different orbital parameters and seasonal insolation may limit the validity of using the Last Interglacial as an analogue for predicted future changes to the climate system. The proxy coverage of the period in the Southern Hemisphere and large parts of the Northern Hemisphere is poor, giving a bias towards Europe and the North Atlantic region. The value of the Last Interglacial in aiding prediction of future climate change is restricted to mainly Europe and the North Atlantic. To improve our ability to predict the potential impacts of future climate change it is essential that the search for proxy data in new locations with a more comprehensive geographical spread are sought out in order to get a more global picture of ancient climate with better resolution.
Acknowledging the problems and benefits of using the Last Interglacial as an analogue for a future warming climate begs the question of whether there may be a more suitable analogue in the past. Loutre and Berger (2002) suggest that Marine Isotope Stage 11 (MIS 11) from 405 to 340 ka would make a better analogue for future climate than the Last Interglacial, due to it being a warmer interglacial period, but with an orbital insolation signal that correlates closely with the recent past and future, giving a much better comparison of orbital forcing. MIS 11 also appears to have been possibly the warmest and longest interglacial of the past 5 million years and had an extended period with little or no continental ice, which is projected to occur under some future global warming scenarios.
Because the observed and predicted rate of increase in greenhouse gas concentrations and global warming is faster than seems to have happened during the Last Interglacial may mean that we are heading into uncertain territory.
I would like to acknowledge the enormous contribution of my fellow SkS contributor, jg, towards the production of this series. Thank you, jg!
derision they heep on anyone that reaches a different conclusion from the warming belief. It is not anti-science to reach a different conclusion, but it is anti-science to close your mind to the idea that you might be wrong. I remain open to that possibility, but I suspect thatfew readers of this website have an open mind to the idea that they might be wrong.[DB] "Although the study of paleoclimate is interesting, we can derive so little of use from these studies, apart from saying that conditions were different."
Actually, climate science derives a great deal of understanding of the present climate through its studies of the past, in particular the paleoclimate.
"Disregarding the orbital characteristics can we even know what the TSI was in the past."
Umm. no. Essentially you are goalpost-shifting here. This new tangent more properly belongs on the A detailed look at climate sensitivity thread.
"It seems that nothing is coincident to todays conditions and therefore any predictions on future climate based on the past must be considered as speculation."
Shorter version: 'Because I haven't read enough of the literature everything I don;t know is speculation.' I.e., handwaving dismissal.
"We need to study the behaviour of the oceans more with regard to the present position of the land masses if we wish to gain any predictive skill."
Climate scientists are doing this very thing, in great detail. But very much off-topic for this thread.
Essentially, your entire comment amounts to 'It's not Bad'. So as you were advised before, please take your concerns to the It’s not bad thread.
You should compare (silently) the type of 'savaging' that goes on at denial sites to the comments typically seen here. You will note, if you take the time to read objectively, that comments here are about the science, the data, the work done - not the individuals who do the work. If you object to such critical discussion of science, why are you here?
Please confine your future commentary to issues of science; unsubstantiated criticism of the people who post on this site will be snipped or deleted wholesale.
warmistscientific point of view." There, fixed it for you. Funny you should resort to calling names in the same post as complaining about name-calling. That you compare this site to WUWT in terms of content and treatment speaks volumes about your ability to think critically, or discern abuse from scientific criticism. Spencer, Christy and Lindzen have made a great many errors, slip-ups, crocks and presented a good number of illusions too. Some articles here call them on that. 'Constructive dialogue' can only start when these people stop misinforming the public about the science of climate change, using all sorts of tired myths. Nobody has presented a scientific case as to why Mann is wrong, and his work has been repeatedly independently verified - do you condone the unjustified abuse heaped upon him at WUWT and elsewhere? Skeptics perpetually fail to understand the consequences of Mann being wrong: that this would mean climate sensitivity is even higher than previously thought! Oops. By the way, the day that a climate skeptic procides a sound scientific case for any of "CO2 isn't the main driver of global warming", "Warming isn't having negative consequences for food production, severe weather, coastal communities etc", "the oceans are not acidifying or sea level rising", then I will break out the champagne, I'd love to be wrong. But I need a sound science case for it, not all the mutually incompatible and easily-debunked myths that seem to be the best the skeptics can come up with. Despite the fact that the big hydrocarbon producers could easily fund just about any scientific study they liked, from Antarctica to the Marianas Trench, with their loose change, not to mention their logistical capabilities, and thus scientifically demonstrate that their product is not polluting the atmosphere, they haven't managed to do so. Inconvenient?I remember an article in SKS where it was stated that the orbital forcing did not explain the warming coming up out of the last ice age. It (orbital forcing) supposedly would only explain about 10% of the warming. The rest would be positive feedbacks I assume. Loss of albedo and co2 coming up out of the oceans. I would like some help getting to that article that explains just that ratio. It helps to emphasize that a small nudge of the climate can make a much bigger response.
renewable guy - See this post by Chris Colose: Milankovitch Cycles.
renewable guy: See also Shakun et al. Clarify the CO2-Temperature Lag.
Rob and Tom,
Thanks. :)