Posts Tagged ‘ice age’

Earth’s temperature over the past two million years

October 6, 2016

A new reconstruction of global average temperature over the past two million years has recently appeared in Nature (Snyder, 2016). That is quite a feat and a first for this duration. The figure below, made by Jos Hagelaars, shows Snyder’s temperature reconstruction, combined with the observed warming since 1880 and projected warming until the year 3000 for two IPCC scenarios, RCP6.0 and RCP8.5.

snyder-et-al-2016-rcp8-5-rcp6-0-nr3

The RCP8.5 can be viewed as a “no mitigation” scenario, whereas RCP6.0 would be a “limited mitigation” scenario. It is clear that in both scenarios global warming over the next centuries will take us out of the temperature realm of the past two million years. A similar figure (which I tweeted yesterday) but then with temperature projections stopping in the year 2100 can be found here.

Even though lauded as a very valuable and novel contribution to the field, Snyder’s reconstruction has also been criticized because the temperature amplitude between glacial and interglacial states appears relatively large (~6 degrees) compared to other recent reconstructions, e.g. by Shakun et al (2012) (~4 degrees). Somewhat related, Snyder estimates the global average temperature during the previous interglacial (Eemian) to be warmer than now, whereas e.g. Hansen et al (2016, under review) argue that they are similarly warm. By the way, sea levels were 6 to 9 metres higher in the Eemian than now. Sea level responds very slowly to a change in temperature, yet another sign of the vast inertia in the climate system.

Shakun_Marcott_HadCRUT4_A1B_Eng

Somewhat overshadowing the actual temperature reconstruction that Snyder presented was her calculation of an earth system sensitivity (ESS) based on a correlation between temperature and CO2 over the past few glacial cycles. The earth system sensitivity denotes the long-term temperature response to a doubling in CO2 concentrations, including e.g. the response of ice sheets (which is typically excluded from the more often used equilibrium climate sensitivity, ECS). She then applied the ESS value of a whopping 9 degrees, obtained from this simple correlation, to the current warming, stating in the abstract:

This result suggests that stabilization at today’s greenhouse gas levels may already commit Earth to an eventual total warming of 5 degrees Celsius (range 3 to 7 degrees Celsius, 95 per cent credible interval) over the next few millennia as ice sheets, vegetation and atmospheric dust continue to respond to global warming.

Where “commit” means that this level of warming would be eventually expected based on current CO2 concentrations.

As Gavin Schmidt wrote, this is simply wrong.

The reason why I think it’s wrong is that in her calculation of ESS she takes the radiative forcing caused by albedo changes (resulting from the massive change in ice coverage between a glacial and interglacial state) and assumes it to be a feedback on the CO2 induced temperature-change.

There are two issues with this:

1) In reality both the changes in albedo (reflectivity) and CO2 concentration are feedbacks on the orbital forcing, and the relation in the one direction (a change in earth’s orbit causing a temperature change which in turn causes albedo and CO2 levels to change) is not necessarily the same as the relation in the reverse direction, as is currently happening with human-induced increases in CO2. Gavin Schmidt makes this point in two consecutive posts at RealClimate (here and here), though you might also want to read Hansen’s take, who has used a similar approach as Snyder did).

2) The ESS value obtained would (ignoring the more complex first point) perhaps be applicable to a glacial-interglacial transition, but decidedly not to an interglacial-‘hyperinterglacial’ transition, where the ice-albedo feedback would of course be much smaller because of the much smaller ice-covered surface area.

This second point was also made by James Annan in response to Hansen’s 2008 Target CO2 paper, where he essentially used the same method as Snyder is using (but came to a smaller ESS value of 6 degrees, because Snyder uses a greater temperature-amplitude between glacial-interglacial). Hansen noted in his paper though that “The 6°C sensitivity reduces to 3°C when the planet has become warm enough to lose its ice sheets.”

In other words, using Snyder’s very (and probably too) high ESS value to project future warming is unwarranted and wrong.

The two epochs of Marcott

March 19, 2013

Guest post by Jos Hagelaars. Dutch version is here.

The big picture (or as some call it: the Wheelchair): Global average temperature since the last ice age (20,000 BC) up to the not-too distant future (2100) under a middle-of-the-road emission scenario.

Shakun_Marcott_HadCRUT4_A1B_Eng

Figure 1: The temperature reconstruction of Shakun et al (green – shifted manually by 0.25 degrees), of Marcott et al (blue), combined with the instrumental period data from HadCRUT4 (red) and the model average of IPCC projections for the A1B scenario up to 2100 (orange).

Earlier this month an article was published in Science about a temperature reconstruction regarding the past 11,000 years. The lead author is Shaun Marcott from Oregon State University and the second author Jeremy Shakun, who may be familiar from the interesting study that was published last year on the relationship between CO2 and temperature during the last deglaciation. The temperature reconstruction of Marcott is the first one that covers the entire period of the Holocene. Naturally this reconstruction is not  perfect, and some details will probably change in the future. A normal part of the scientific process.

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Past, current and future sea level rise

January 18, 2011

What’s there to worry about sea level rise; it’s going very slowly, right? Let’s put current sea level rise in a historical perspective.

Here’s a graph of sea level since the last ice age. As the ice from the last ice age was melting, sea levels rose by some 120 metres over the course of about 8000 years, before it flattened out ~6000 years ago. On the top right, I drew a black line with an approximate slope of 3 mm/year, which is the current rate of sea level rise (over the past 20 years or so). This is much faster than the relatively stable sea level during the ~6000 years before, though not as fast as the sea level rise at the end of the last ice age.

Let’s zoom in on the last 9000 years (covering most of the Holocene epoch). The strong sea level rise at the end of the last ice age is still visible on the left hand side, slowing down 7000 years ago and even more so 4000 years ago. Until recently: Current sea level rise represents a clear increase. For the future, most recent estimates of sea level rise fall between 0.5 and 1.5 metres in 2100. It won’t stop thereafter, since there’s a lot of inertia involved in warming up the oceans and in melting (parts of) the large ice sheets (Greenland and Antarctica).

 

Under a business as usual scenario sea level will likely rise by multiple metres over the course of the next few centuries (see e.g. the Dutch DeltaCommittee and German WBGU reports). The vertical red line attempts to give an indication of these long term projections (which are uncomfortably uncertain). With increased warming, sea level rise will likely also increase, so the black line denoting 3 mm/yr can be seen as a lower bound (until the temperature goes down again and the climate system re-equilibrates). An upper bound is harder to predict, because that depends e.g. on tipping points being surpassed, such as parts of the great ice sheets becoming unstable. The fact that in the past sea level has risen with multiple metres per century means that such rates are physically possible. Whether that will happen again in the next millennium depends partly on nature’s forces (uncertainty in the physics, i.e. the response of ice sheets) and partly on what we do to poke nature around (uncertainty in our future emissions).

Quoting from a  recent book “Climate Change: Global Risks, Challenges and Decisions” (eds. Richardson, Steffen and Liverman), the chapter on sea-level rise and ice-sheet dynamics:

The period of relative stability of sea level over the past 6000–7000 years (Harvey and Goodwin, 2004 ) has now ended, and sea level is undoubtedly rising in the post-industrial period (IPCC, 2007a ). Given the massive heat capacity of the ocean, the Earth is already committed to many more centuries of sea-level rise due to thermal expansion alone. The dynamics of the large polar ice sheets and the rapid retreat of glaciers and ice caps will significantly add to the magnitude of sea-level rise. The critical questions are: how much and how fast?

(…) 

Several lines of evidence point towards a sea-level rise by 2100 of perhaps a metre higher than 1990, which is at the upper limit of the IPCC ( 2007a ) range of projections. First, the mid-range of projections of semi-empirical models is centred around 1 m (Rahmstorf, 2007; Grinsted et al ., 2009 ). Second, observed sea-level rise is currently tracking at or near the upper limit of the IPCC projections (Rahmstorf et al ., 2007 ; Domingues et al ., 2008 ). Third, recent observations show an increasing rate of mass loss from both the Greenland and West Antarctic ice sheets (Cazenave, 2006 ; Rignot and Kanagaratnam, 2006 ). Finally, an analysis of the kinematic constraints on dynamical ice loss suggests a plausible increase in sea level of 0.8 m by 2100 (Pfeffer et al ., 2008 ).

Nevertheless, considerable uncertainty surrounds many of the processes that contribute to sea-level rise, especially concerning the dynamics of the large polar ice sheets. All of these uncertainties point in the same direction – towards more rapid and severe sea-level rise. Thus, rises higher than 1 m by 2100 cannot be ruled out.

(Graphs based on Robert Rodhe’s Global Warming Art)


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