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Wednesday, January 19, 2011

"Paleoclimate Implications for Human-Made Climate Change" by James E. Hansen and Makiko Sato

Dr. Hansen has sent this draft out and welcomes suggestions:

Paleoclimate Implications for Human-Made Climate Change
James E. Hansen and Makiko Sato

NASA Goddard Institute for Space Studies and Columbia University Earth Institute, New York, NY, U.S.A.


Milankovic climate oscillations help define climate sensitivity and assess potential human-made climate effects.  We conclude that Earth in the warmest interglacial periods was less than 1 °C warmer than in the Holocene and that goals of limiting human-made warming to 2 °C and CO2 to 450 ppm are prescriptions for disaster. Polar warmth in prior interglacials and the Pliocene does not imply that a significant cushion remains between today's climate and dangerous warming, rather that Earth today is poised to experience strong amplifying polar feedbacks in response to moderate additional warming.  

Deglaciation, disintegration of ice sheets, is nonlinear, spurred by amplifying feedbacks.  If warming reaches a level that forces deglaciation, the rate of sea level rise will depend on the doubling time for ice sheet mass loss.  Gravity satellite data, although 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.  The emerging shift to accelerating ice sheet mass loss supports our conclusion that Earth's temperature has returned to at least the Holocene maximum.  Rapid reduction of fossil fuel emissions is required for humanity to succeed in preserving a planet resembling the one on which civilization developed.

1.  Introduction

Climate change is likely to be the predominant scientific, economic, political and moral issue of the 21st century.  The fate of humanity and nature may depend upon early recognition and understanding of human-made effects on Earth's climate (Hansen, 2009). Tools for assessing the expected climate effects of alternative levels of human-made changes of atmospheric composition include (1) Earth's paleoclimate history, showing how climate responded in the past to changes of boundary conditions including atmospheric composition, (2) modern observations of climate change, especially global satellite observations, coincident with rapidly changing human-made and natural climate forcings, and (3) climate models and theory, which aid interpretation of observations on all time scales and are useful for projecting future climate under alternative climate forcing scenarios.

This paper emphasizes information provided by paleoclimate data.  Milankovic climate oscillations, the glacial-interglacial climate swings associated with perturbations of Earth's orbit, provide a precise evaluation of equilibrium climate sensitivity, i.e., the response to changed boundary conditions after the atmosphere and ocean have sufficient time to restore planetary energy balance.  Implications become clearer when Pleistocene climate oscillations are viewed in the context of larger climate trends of the Cenozoic Era.  Ice cores and ocean cores are complementary tools for understanding, together providing a more quantitative assessment of the dangerous level of human interference with the atmosphere and climate.

Fig. 1 shows estimate global deep ocean temperature over the past 65.5 million years, the Cenozoic Era.  The deep ocean temperature is inferred from a global compilation of oxygen isotopic abundances in ocean sediment cores (Zachos et al., 2001), with the temperature estimate extracted from oxygen isotopes via the simple approximation of Hansen et al. (2008).

This deep ocean temperature change is similar to global surface temperature change, we
will argue, until the deep ocean temperature approaches the freezing point of ocean water.  Thus late Pleistocene glacial-interglacial deep ocean temperature changes (Fig. 1c) are only about two thirds as large as global mean surface temperature changes.

In this paper we discuss Cenozoic climate change and its relevance to understanding of human-made climate change.  We review how Milankovic climate oscillations provide a precise measure of climate sensitivity to any natural or human-made climate forcing.  We summarize how temperature is extracted from ocean cores to clarify the physical significance of this data record, because, we will argue, ocean core Milankovic data have profound implications about the dangerous level of human-made interference with global climate.  Finally we discuss the temporal response of the climate system to the human-made climate forcing.

2.  Cenozoic Climate Change

The Cenozoic era illustrates the huge magnitude of natural climate change.  Earth was so warm in the early Cenozoic that polar regions had tropical-like conditions – indeed, there were alligators in Alaska (Markwick, 1998).  There were no large ice sheets on the planet, so sea level was about 75 meters higher than today.

Earth has been in a long-term cooling trend for the past 50 million years (Fig. 1a). By approximately 34 Mya (million years ago) the planet had become cool enough for a large ice sheet to form on Antarctica. Ice and snow increased the albedo ('whiteness' or reflectivity) of that continent, an amplifying feedback that contributed to the sharp drop of global temperature at that time.  Moderate warming between 30 and 15 Mya was not sufficient to melt all Antarctic ice. The cooling trend resumed about 15 Mya and accelerated as the climate became cold enough for ice sheets to form in the Northern Hemisphere and provide their amplifying feedback.

The Cenozoic climate changes summarized in Fig. 1 contain insights and quantitative information relevant to assessment of human-made climate effects. Carbon dioxide (CO2) plays a central role in both the long-term climate trends and the short-term oscillations that were magnified as the planet became colder and the ice sheets larger.  Cenozoic climate change is discussed by Zachos et al. (2001), IPCC (2007), Hansen et al. (2008), and many others.  We describe here implications about the role of CO2 in climate change and climate sensitivity.

CO2 is the principal forcing that caused the slow Cenozoic climate trends over millions of 
years, as the solid Earth (volcanic) source altered the amount of CO2 in surface carbon reservoirs (atmosphere, ocean, soil and biosphere).  CO2 is also a principal factor in the short-term climate oscillations that are so apparent in parts (b) and (c) of Fig. 1.  However, in these glacial-interglacial oscillations atmospheric CO2 operates as a feedback: total CO2 in the surface reservoirs changes little on these shorter time scales, but the distribution of CO2 among the surface reservoirs changes as climate changes.  As the ocean warms, for example, it releases CO2 to the atmosphere, providing an amplifying climate feedback that causes further warming.

The fact that CO2 is the dominant cause of long-term Cenozoic climate trends is obvious from consideration of Earth's energy budget.  Such large climate changes cannot result from redistribution of energy within the climate system, as might be caused by changes of atmosphere or ocean dynamics.  Instead a substantial global climate forcing is required.  The climate forcing must be due to a change of energy coming into the planet or changes within the atmosphere or on the surface that alter the planet's energy budget.

Solar luminosity is increasing on long time scales, as our sun is at an early stage of solar evolution, "burning" hydrogen, forming helium by nuclear fusion, slowly getting brighter.  The sun's brightness increased steadily through the Cenozoic, by about 0.4 percent according to solar physics models (Sackmann et al., 1993).  Because Earth absorbs about 240 W/m² of solar energy, that brightness increase is a forcing of about 1 W/m²  This small linear increase of forcing, by itself, would have caused a modest global warming through the Cenozoic Era.

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1 comment:

Leon said...

This world changing article deserves more attention.

Oxygen isotope analysis has been falsely held for global past temperature analysis. All it tells you is the regional ocean temperature and global water/ice distribution. These are partly determined by ocean currents.

I would note that man made warming has already passed the 1 degree threshold. Anthropogenic aerosol cooling decreases the warming effect. This means humanity will not succeed in preserving a planet resembling the one on which civilisation developed.