Earth’s temperature over the past two million years

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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.

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5 Responses to “Earth’s temperature over the past two million years”

  1. thomaswfuller2 Says:

    Why do you treat the Representative Concentration Pathways as projections of potential realities? They are not.

    As I’m sure you are aware, they are ad hoc explanations of possible ways the planet could achieve pre-assigned levels of radiative forcing. Those levels were given as a starting point in the exercise to develop inputs to climate models.

    They are not projections. The researchers have specifically said they are not projections for over a decade. A casual look at the ad hoc explanations they came up with show that they are not at all serious, especially RCP 8.5.

    Worse, RCP 8.5 has already been invalidated by choices we have collectively made about green energy, emissions control and population.

    Only those wishing to foster a sense of panic would hold RCP 8.5 out as either a carrot or stick for influencing policy.

    You wouldn’t do that, would you Bart?

  2. Jos Hagelaars Says:

    “Why do you treat the Representative Concentration Pathways as projections of potential realities? They are not.”
    “RCP 8.5 has already been invalidated by choices we have collectively made about green energy, emissions control and population.”

    To me the RCP’s are clearly described by Bart as “scenario’s” and the RCP8.5 scenario is described as a “no mitigation” scenario.”. Indeed agreements are made about CO2-emission control so hopefully we will not follow that path. But you never know, there are still people in some governments who think climate change is a hoax. And we are still quite close to RCP8.5:

  3. wert Says:

    TWF: You wouldn’t do that, would you Bart?

    I agree with Thomas here, presenting RCP8.5 as no-mitigation future is an untrue statement. Please come back and tell you didn’t mean this.

  4. Bob Brand Says:

    As Thomas Fuller mentions:

    Worse, RCP 8.5 has already been invalidated by choices we have collectively made about green energy, emissions control and population.

    So it is actually mitigation (“emissions control”) which “invalidates” RCP8.5. By extension, without any mitigation RCP8.5 might be a valid projection — quite possibly even on the low side if one would consider only CO2 emissions.

    A point of criticism on RCP8.5 is that the net methane emissions are rather high, which impacts CO2-equivalent and it is the reason why the graph shows “>1000ppm CO2eq”.

    On the other hand, the RCP8.5 scenario already contains some plausible developments which would happen to limit CO2 emissions even without mitigation policies, such as an increasing role for natural gas and nuclear.

    See Riahi et al 2011:

    This paper summarizes the main characteristics of the RCP8.5 scenario. The RCP8.5 combines assumptions about high population and relatively slow income growth with modest rates of technological change and energy intensity improvements, leading in the long term to high energy demand and GHG emissions in absence of climate change policies.

    http://link.springer.com/article/10.1007/s10584-011-0149-y

  5. carl zorn Says:

    The graph of earths temperature over the last two million years is very telling. The peaks of both high and low temperatures show that a change in the energy expended in our atmosphere changed suddenly. It can’t all be caused by human activity because we weren’t around.

    The trend of movement of the magnetic poles indicate that there is a large change occurring in our planet. Why do the magnetic poles move? The earths core is not solid but flows around a more solid iron core. This movement of the material within the mantle around the iron core generates the magnetic field around the planet. The gigawatts of energy generated expands the magnetic field around the planet and beyond, well into the ionsphere.

    The plasma-sphere is not really spherical but a doughnut-shaped region (a torus) with the hole aligned with Earth’s magnetic axis. [In this case the use of the suffix -sphere is more in the figurative sense of a “sphere of influence”.] The Earth’s plasmasphere is made of just that, a plasma, the fourth state of matter. This plasma is composed mostly of hydrogen ions (protons) and electrons. It has a very sharp edge called the plasmapause. The outer edge of this doughnut over the equator is usually some 4 to 6 Earth radii from the center of the Earth or 12,000-20,000 miles (19,000-32,000 km) above the surface. The plasmasphere is essentially an extension of the ionosphere. Inside of the plasmapause, geomagnetic field lines rotate with the Earth. The inner edge of the plasmasphere is taken as the altitude at which protons replace oxygen as the dominant species in the ionospheric plasma which usually occurs at about 600 miles (1000 km) altitude. The plasmasphere can also be considered to be a structure within the magnetosphere.

    The rotation of the geomagnetic field lines in the plasma-sphere in this proton plasma has a feedback effect on the magnetic field of the earth. This feedback results in additional energy applied to the movement of the magma around the earths iron core. The size of the interface between the suns radiation and the plasma-sphere generates an enormous amount of energy that is amplifies the movement of the magma that is generating the magnetic field.

    The position of the magnetic pole is evidence of the relative speed of the magma. A higher speed results in an offset from the center, just like a field in a generator. With the movement of the magnetic pole towards the center of the earths pole, it is evidence of a slowing magma flow in the earth. This causes a reduction in the magnetic force generating the magnetosphere.

    The reduction in the magnetosphere force allows further penetration of suns energy into the atmosphere resulting in additional heating of the earths atmosphere.

    The really interesting thing about the graph of the last two million years of global temperature is the peaks and valleys. I think this indicates that there was a sudden change in magma flow and relative excitation of the earths atmosphere. This can only be caused by volcanic activity. Volcanoes can emit large amounts of water vapor, CO2, and sulfuric compounds into the upper atmosphere. These particles are bombarded by the solar radiation and separated into ions which would add material to the plasma-sphere and cause a change in excitation of the magnetosphere.

    As I read more about the changes in the magnetic poles, and the corresponding changes in the atmospheric temperatures, it isn’t unreasonable to expect the reduction in magma flow will cause a volcanic eruption cycle to erupt, due to the change in pressure and temperature exerted on the crust by the magma. It looks like we are almost at a peak in temperature so shouldn’t we expect a change in volcanic activity in the next couple of centuries?

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