ScienceWeek

CLIMATE CHANGE: THE SCIENTIFIC CONSENSUS

The following points are made by Naomi Oreskes (Science 2004 306:1686):

1) Policy-makers and the media, particularly in the US, frequently assert that climate science is highly uncertain. Some have used this as an argument against adopting strong measures to reduce greenhouse gas emissions. For example, while discussing a major US Environmental Protection Agency report on the risks of climate change, then-EPA administrator Christine Whitman argued, "As [the report] went through review, there was less consensus on the science and conclusions on climate change" [1]. Some corporations whose revenues might be adversely affected by controls on carbon dioxide emissions have also alleged major uncertainties in the science [2]. Such statements suggest that there might be substantive disagreement in the scientific community about the reality of anthropogenic climate change. This is not the case.

2) The scientific consensus is clearly expressed in the reports of the Intergovernmental Panel on Climate Change (IPCC). Created in 1988 by the World Meteorological Organization and the United Nations Environmental Programme, IPCC's purpose is to evaluate the state of climate science as a basis for informed policy action, primarily on the basis of peer-reviewed and published scientific literature [3]. In its most recent assessment, IPCC states unequivocally that the consensus of scientific opinion is that Earth's climate is being affected by human activities: "Human activities ... are modifying the concentration of atmospheric constituents ... that absorb or scatter radiant energy. ... [M]ost of the observed warming over the last 50 years is likely to have been due to the increase in greenhouse gas concentrations" (p. 21 in [4]).

3) IPCC is not alone in its conclusions. In recent years, all major scientific bodies in the US whose members' expertise bears directly on the matter have issued similar statements. For example, the National Academy of Sciences report, Climate Change Science: An Analysis of Some Key Questions, begins: "Greenhouse gases are accumulating in Earth's atmosphere as a result of human activities, causing surface air temperatures and subsurface ocean temperatures to rise" {p. 1 in [5]). The report explicitly asks whether the IPCC assessment is a fair summary of professional scientific thinking, and answers yes: "The IPCC's conclusion that most of the observed warming of the last 50 years is likely to have been due to the increase in greenhouse gas concentrations accurately reflects the current thinking of the scientific community on this issue" (p. 3 in [5]). Others agree. The American Meteorological Society, the American Geophysical Union, and the American Association for the Advancement of Science (AAAS) all have issued statements in recent years concluding that the evidence for human modification of climate is compelling.

References (abridged):

1. A. C. Revkin, K. Q. Seelye, New York Times, 19 June 2003, A1

2. S. van den Hove, M. Le Menestrel, H.-C. de Bettignies, Climate Policy 2 (1), 3 (2003)

3. See www.ipcc.ch/about/about.htm.

4. J. J. McCarthy et al., Eds., Climate Change 2001: Impacts, Adaptation, and Vulnerability (Cambridge Univ. Press, Cambridge, 2001)

5. National Academy of Sciences Committee on the Science of Climate Change, Climate Change Science: An Analysis of Some Key Questions (National Academy Press, Washington, DC, 2001)

Science http://www.sciencemag.org

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CARBON DIOXIDE AND CLIMATE CHANGE

Notes by ScienceWeek:

The term "greenhouse effect" refers to the blockade of longwave (infrared) radiation from the Earth into space by trace constituents of the atmosphere, primarily water vapor, carbon dioxide, ozone, methane, nitrous oxide, and various chlorofluorocarbons -- gases referred to collectively as the "greenhouse gases". The sensitivity of the biosphere to the greenhouse effect has existed throughout the history of life on Earth: the infrared radiation-blocking gases return radiated heat to the ground, accounting for approximately 70 percent of the net input of energy to the Earth's surface.

In this context, the term "radiative forcing" refers in general to climate changes produced by changes in incoming or outgoing radiative heat to and from the Earth's surface.

The term "tectonic" refers in general to deformations of the Earth's crust and consequent structural changes, and "tectonic timescales" are the long timescales (e.g., tens of millions of years) over which such deformations occur.

The following points are made by T. J. Crowley and R.A. Berner (Science 2001 292:870):

1) It has long been known that on timescales of tens of millions of years, intervals of continental glaciation were interspersed with intervals of little or no ice. The magnitude of warmth during these warm intervals is impressive: for example, at times during the Cretaceous period (approximately 65 to 145.6 million years ago) duck-billed dinosaurs roamed the northern slope of Alaska, and deep and bottom waters of the ocean, now near freezing, could reach a balmy 15 degrees celsius.

2) In the 1980s, a convergence of results from paleoclimate data and geochemical and climate models suggested that such long-term variations in climate were strongly influenced by natural variations in the carbon dioxide content of the atmosphere. Recently, some geochemical results have raised concerns about the validity of this conclusion, since carbon dioxide concentrations over the past 65 million years appear to have reached low levels well before the most recent phase (the past 3 million years) of Northern Hemisphere glaciation. A study spanning the Phanerozoic time-frame (the past 570 million years) also suggests some decoupling between times of predicted high carbon dioxide and some climate indices.

3) In order to reevaluate the validity of the assumed carbon dioxide-climate link, the authors compared estimates of Phanerozoic carbon dioxide variations and net radiative forcing with the continental glaciation record and low-latitude temperature estimates. The authors report that the first order agreement between the carbon dioxide record and continental glaciation continues to support the conclusion that carbon dioxide has played an important role in long-term climate change.

4) The authors conclude that to weigh the merits of the carbon dioxide paradigm, it may be necessary to expand the scope of climate modeling. For factors responsible for the presence or absence of continental ice, the carbon dioxide model works very well. In contrast, there are substantial gaps in our understanding of how climate models distribute heat on the planet in response to carbon dioxide changes on tectonic time scales. "Given the need for better confidence in some of the paleoclimate data, and unanticipated complications arising from altered tectonic boundary conditions, it may be hazardous to infer that existing discrepancies between models and data cloud interpretations of future anthropogenic greenhouse gas projections."

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CLIMATOLOGY: ON CLIMATE CHANGE

The following points are made by T.J. Osborn and K.R. Briffa (Science 2004 306:621):

1) How sensitive is the climate to changes in solar irradiance, atmospheric aerosols, greenhouse gases, and other climate forcings? To answer this question, we first need to know the true extent of past climate fluctuations. The changing temperatures over past centuries and millennia have been reconstructed by regressing annually resolved climate proxy records -- for example, from ice cores and tree rings -- against recent thermometer measurements. An important related question is whether climate changes over decades and longer are likely to have been captured realistically in such reconstructions of Northern Hemisphere (NH) mean temperature.(1)

2) The likelihood that reconstructions of this kind represent accurate "hindcasts" of past climate is usually assessed by verification against a short period of independent thermometer data. Such verification is only possible for short-term (annual to decadal) climate variability, because the instrumental climate record is too short to sample longer (decadal to centennial) time scales adequately.

3) To overcome this limitation, von Storch et al(1) used a 1000-year simulation from a coupled ocean-atmosphere model as a test-bed in which the (simulated) NH temperature is known. They then generate pseudo-proxy records by sampling a small selection of the model's simulated grid-box temperatures (replicating the spatial distribution of existing proxy records) and degrading them with statistical noise. The authors demonstrated that most of their attempts to reconstruct the model's NH temperature with the pseudo-proxies result in significant underestimates of the amplitude of fluctuations over the last millennium. Published temperature reconstructions for the real world, based on similar calibration methods, may suffer from the same limitation.

4) Although von Storch et al(1) focused their discussion on the reconstruction method of Mann et al(2), their conclusions are relevant to other attempts to reconstruct NH temperature history. They demonstrated even greater loss of long-term variations with a simple regression-and-averaging method [this observation was also made in (3)]. The results may apply to all regression-based methods. Accepting the results of von Storch et al(1) does not mean that we must also accept that their simulated temperature history is close to reality -- merely that it is a reasonable representation of climate behavior for which any valid reconstruction method should perform adequately.(4,5)

References (abridged):

1. H. von Storch et al., Science 306, 679 (2004)

2. M. E. Mann, R. S. Bradley, M. K. Hughes, Nature 392, 779 (1998)

3. T. J. Osborn, K. R. Briffa, F. H. Schweingruber, P. D. Jones, www.cru.uea.ac.uk/~timo/p/a/

4. K. R. Briffa et al., J. Geophys. Res. 106, 2929 (2001)

5. P. D. Jones, M. E. Mann, Rev. Geophys. 42, 2003RG000143 (2004)

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