, and , 2012: Paleoclimate implications for human-made climate change. In Climate Change: Inferences from Paleoclimate and Regional Aspects. A. Berger, F. Mesinger, and D. Šijački, Eds. Springer, pp. 21-48, doi:10.1007/978-3-7091-0973-1_2.
Paleoclimate data help us assess climate sensitivity and potential human-made climate effects. We conclude that Earth in the warmest interglacial periods of the past million years was less than 1°C warmer than in the Holocene. Polar warmth in these interglacials and in the Pliocene does not imply that a substantial cushion remains between today's climate and dangerous warming, but rather that Earth is poised to experience strong amplifying polar feedbacks in response to moderate global warming. Thus goals to limit human-made warming to 2°C are not sufficient — they are prescriptions for disaster. Ice sheet disintegration is nonlinear, spurred by amplifying feedbacks. We suggest that ice sheet mass loss, if warming continues unabated, will be characterized better by a doubling time for mass loss rate than by a linear trend. Satellite gravity data, though too brief to be conclusive, are consistent with a doubling time of 10 years or less, implying the possibility of multi-meter sea level rise this century. Observed accelerating ice sheet mass loss supports our conclusion that Earth's temperature now exceeds the mean Holocene value. Rapid reduction of fossil fuel emissions is required for humanity to succeed in preserving a planet resembling the one on which civilization developed.
We are done! Toast! Game over! Hansen and Sato have determined that the Earth is currently less than 1 degree cooler than the warmest periods in the past MILLION years and that "Rapid reduction in the use of fossil fuels is required for humanity to succeed in preserving a planet resembling the one on which civilization developed"
The first obvious point is with 6 plus billion humans on the planet, could it possibly "resemble" the planet before the 6 plus billion? No. 6 Plus billion humans have altered the planet. If a planet resembling the Earth at the dawn of civilization is what is needed, we are F_ked.
From the abstract, Hansen and Sato would seem to be confident we are F_ked and the "ONLY" solution is to stop using fossil fuels now.
From the body of the paper, "We conclude that ocean cores provide a better measure of global temperature change than ice cores during those interglacial periods that were warmer than the pre-industrial Holocene." HS provide a number of issues with ocean cores, but generally ocean core do seem to provide a better measure of past climate than surface temperatures which are recorded in ice cores and compared to "global average surface temperature".
This chart of the "normalized" data of ocean cores versus the CO2 recorded in the Antarctic ice cores just compares the timing events for the past 50 thousand years. The warming of the oceans recorded in the Lea et al. Galapagos reconstruction appears to precede the Antarctic CO2 increase and the higher northern latitude Tdo, temperature of deep ocean and Tsurf, temperature of the ocean surface in the reconstruction by Bintanji and Van de Wal (2005), all of the data available at the ncdc.noaa.gov/paleo website.
Note that the Antarctic CO2 starts a rise circa 7.5ka BP. That could indicate that man began having an impact on climate well before the discovery of the internal combustion engine, electricity and science, but around the time of the discovery of agriculture. One on the greater uncertainties is the impact that mankind's agricultural activities would have had on the land based glacial area and persistent snow fields. Snow and ice is not conducive to agriculture, but the melt water is desirable. Agricultural activity, burning used in slash and burn agriculture, provides darker ash which tend to speed up snow and ice melt. Erosion due to wind following slash and burn or more modern plowing provide darker dust and debris that would fall on snow and ice enhancing snow and ice melt. Plus, spreading or broadcasting manure, peat dust and ash on spring snow enhances snow and ice melt. With man industriously using the simple tool of fire, man would have a large impact on the area and amount of ice and snow that hampers or helps his agricultural pursuits. This would be land use change impacting climate not fossil fuel emissions.
The conundrum that Hansen and Sato face is that the impact of that land use predating the instruments used to build a "global surface temperature" is a significant unknown. In the paper HS mentions the large non-linear impact of snow and ice albedo. "However, there is one feature in the surface albedo versus temperature scatter plots (Figs. 3e and 3f) that seems unrealistic: the tail at the warmest temperatures, where warming of 1°C produces no change of sea level or surface albedo." A little issue like responses that "seem unrealistic" might cause some to be less certain in their conclusions.
It could be that over emphasizing the radiant forcing without properly considering the less radiant based internal thermodynamics, could lead to questionable conclusions. Now that HS have "discovered" that ocean paleo appears to provide a more reliable picture of past climate, perhaps changes in the ocean currents that would impact internal heat distribution need to be considered differently.
Toggweilder and Bjornsson (2000) using an ocean model conclude, "The results here suggest that much of the full thermal effect of Drake Passage could have been realised well before the channel was very wide or very deep. This is because the mere presence of an open gap introduces an asymmetry into the system that is amplified by higher salinities in the north and lower salinities in the south. This kind of haline effect, and the possibility of increased Antarctic sea-ice and land-ice, lead us to conclude that the thermal response to the opening of Drake Passage could have been fairly abrupt and quite large, perhaps as large as the 4–5°C cooling seen in palaeoceanographic observations." The opening of the Drake Passage changed the way the oceans disperse and accumulation energy. Since radiant impacts must assume limited advection, changing the rate and pattern of advection of the primary source of energy that responds to atmospheric forcing, would impact the efficiency of the atmosphere to retain energy. Since that surface, the oceans, provides the energy that the atmosphere retains, it would seem prudent to compare apples to apples.
This chart compares the HadSST2 version of the northern and southern hemisphere sea surface temperatures. These "surface" temperatures are not the true ocean "surface" but typically sub-surface temperatures collected by ship engine raw water intakes and bucket samples during the sail era. The total warming is similar to the "global surface" warming since the sub-surface ocean data makes up some 70% of the "global surface" record.
This chart uses the Berkeley Earth Surface Temperature (BEST) data Tmin for the northern and southern hemispheres with the same smoothing. Note the SH-NH difference in both charts. There is more variation in the SST than the Tmin, but both show a shift beginning roughly at 1985. In the BEST data, the shift appears to be extraordinary and in the SST data, the shift is not.
There is a very interesting note in the Toggweilder and Bjornsson paper, "It is interesting to note in Fig. 10 that the overturning circulation initiated by an open Drake Passage has very little impact on the magnitude of tropical temperatures. This is because the same volume of cold deep water upwells through the thermocline in low latitudes whether Drake Passage is open or not."
According to the Herbert et al. 2010 tropical oceans temperature reconstruction, the Eastern Pacific which has considerable impact on climate with the ENSO fluctuations, has cooled considerably in the past 4 million years. Globally, the opening of the Drake Passage may not have had much impact on the tropical ocean temperatures, but the response of the Eastern Pacific would seem to indicate that the Drake passage could have considerable impact on the ocean Haline circulation.
As Toggweilder and Bjornsson note, "The results here, based on a coupled model run without restoring boundary conditions, suggest that the impact of an open Drake Passage is larger and more deeply ingrained in the climate system than previously supposed." The global ocean thermal asymmetry would appear to be a problem for a global radiant model that depends on symmetry to have full impact. With the opening of the Drake Passage having an impact of roughly 4 C causing ~3C of NH warming at the expense of ~3C SH cooling, Hansen and Sato's model of climate "seems unrealistic" because "global surface air temperature" is not the reference one should use on a planet covered with water.
The climate of the world has no doubt changed with 6 plus billion mouths to feed, but perhaps there is more to the story than fossil fuels.