Paleoclimate Implications for Human-Made Climate Change

by James E. Hansen and Makiko Sato

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.

Draft paper at this link.

The draft paper has been submitted for publication in the Belgrade Milankovitch Symposium volume -- the paper is now under review.

Doctor James Hansen, an adjunct professor of Earth and Environmental Sciences at Columbia University, heads the NASA Goddard Institute for Space Sciences. His website can be found at: http://www.columbia.edu/~jeh1/