ScienceWeek

ECOLOGY: FUTURE CLIMATE CHANGE AND TERRESTRIAL VEGETATION

The following points are made by Mark Maslin (Science 2004 306:2197):

1) One of the most pressing questions concerning future climate change is how it will affect the terrestrial vegetation [1]. Nowhere is this more hotly debated than in the tropics, where the future of the Amazon rainforest [2] and the continued viability of current agricultural practices [3] are at stake. Studies of past climates may elucidate how quickly vegetation can respond to climate change. Recently, Hughen et al [4] reported that tropical vegetation in Venezuela has in the past responded to climate change within less than 50 years. In contrast, Jennerjahn et al [5] reported a vegetation response time of 1000 to 2000 years in northeast Brazil.

2) Does this difference mean that there is a conflict in our understanding of the effect of climate change on tropical vegetation? On the contrary: The difference between the estimated ecological response times in [4,5] may provide valuable insight into how different parts of the tropics respond to rapid climate changes. As the study by Jennerjahn et al [5] demonstrated, climate thresholds and ecological or vegetation thresholds are not always the same.

3) Usually, comparison between past land and ocean records is difficult, because the errors in dating each record can be larger than the lead and lag times that are being studied. This problem was solved by Jennerjahn et al [5] who, like Hughen et al [4], look at vegetation and climate records in the same marine sediment core. As long as there are no delays in the transport of either the continental or oceanic signal to the marine sediment, the lead and lag times between climate and vegetation can be looked at in detail.

4) Jennerjahn et al [5] studied a marine sediment core from the continental margin off the northeast coast of Brazil. They focused on the last "Heinrich event", a cool period accompanied by ice rafting in the North Atlantic that lasted from about 17,500 to 15,500 years ago. At the beginning of the event, they observed a substantial increase in the amount of iron deposited in their marine core, suggesting a sharp increase in continental erosion from enhanced rainfall [5]. This increased rainfall is accompanied by an increase in pioneer vegetation, as indicated by moss fern spores. A major increase in tree pollen and tree fern spores, showing the development of gallery and montane forest, does not occur until 1000 years later.

5) Today, northeast Brazil has a semiarid climate. A dry period of 8 months per year prevents the development of rainforest, which cannot survive dry periods of more than 4 months. Jennerjahn et al [5] suggest that there may have been a substantial increase in the amount of rainfall and the duration of the wet season at the start of the Heinrich event, but that the dry season still lasted more than 4 months, preventing the development of forest. Only later in the Heinrich event was this ecological threshold of four dry months crossed, allowing the development of forest in northeast Brazil. Different dry season lengths may thus produce a more sensitive and direct relationship between climate and vegetation in Venezuela [4] than in northeast Brazil [5].

References (abridged):

1. M. A. Maslin, Global Warming, a Very Short Introduction (Oxford Univ. Press, Oxford, 2004), p. 162

2. P. M. Cox, R. A. Betts, C. D. Jones, Nature 408, 184 (2000)

3. J. J. McCarthy et al., Eds., Climate Change 2001: Impacts, Adaptation, and Vulnerability, Contribution of Working Group II to the Third Assessment Report of the Intergovernmental Panel on Climate Change (IPCC) (Cambridge Univ. Press, Cambridge, 2001)

4. K. Hughen et al., Science 304, [1955] (2004)

5. T. C. Jennerjahn et al., Science 306, 2236 (2004)

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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)

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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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