Frigg et al. Winsberg and Goodwin , however, contend that more work is needed to establish that the hawkmoth effect is as devastating for probabilistic climate projection as Frigg et al. Stainforth et al. Likewise, Katzav argues that climate change projections can often be interpreted as indicating real possibilities :.
There are also intermediate views. For the moderate emission scenario known as RCP6.
The IPCC thus asserted more than that the changes projected by the CMIP5 ensemble were possibilities, but they did not go so far as to assign a single, full probability density function over temperature change values. Intertwined with the issue of ensemble interpretation is the issue of weighting, i.
This is part and parcel of some methods, but in MME studies it has been common to give equal weight to participating models, as the IPCC approach illustrates. Part of the original motivation for model democracy was the difficulty of determining which of the state-of-the-art models included in MMEs would give the most accurate projections of future conditions see Section 4. This difficulty notwithstanding, recently weighting has been advocated on the grounds that, for at least some projected variables, there is good reason to think that some models are more skillful than others and, moreover, the one-model one-vote approach fails to take account of dependence among models, i.
A key challenge, however, is to select and combine relevant metrics of performance and other criteria to assign appropriate weights for a given projected variable Gleckler et al. The basic issue of interpretation discussed above also remains, i.
There are many other philosophically-interesting questions related to ensemble climate projection Frame et al. First, how can ensemble studies be designed so that they probe uncertainty in desired ways? Second, how significant are robust results? Climate scientists often assume that agreement among projections has special epistemic significance—e. There are also questions about how to quantify the extent of agreement or robustness see Collins et al.
Finally, to what extent do non-epistemic values influence ensemble results? Parker a suggests that this influence might be dampened by representing uncertainty in coarser ways that also better reflect the extent of actual uncertainty, e. The use of these labels varies, however. Contrarians have played a role in creating or sustaining a number of public controversies related to climate science.
Four of these are discussed very briefly below, along with some recent philosophical work reflecting on the impact of contrarian dissent. The Tropospheric Temperature Controversy. Satellites and radiosondes are the primary means of monitoring temperatures in this layer.
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Prima facie, this presented a challenge to climate science, and it became a key piece of evidence in contrarian dismissals of the threat of anthropogenic climate change. Over time, additional research uncovered numerous problems with the satellite and radiosonde data used to estimate tropospheric temperature trends, with many of these problems related to homogenization NRC ; Karl et al.
Nevertheless, the debate continues see, e. The Hockey Stick Controversy. The hockey stick controversy focused on some of the first millennial-scale paleoclimate reconstructions of the evolution of Northern Hemisphere mean near-surface temperature. These reconstructions, when joined with the instrumental temperature record, indicate a long, slow decline in temperature followed by a sharp rise beginning around ; their shape is reminiscent of a hockey stick e.
Contrarian criticism in the published literature followed two main lines. Mainstream climate scientists offered direct replies to these challenges e. Book length accounts of the controversy from opposing sides include Montford and Mann The Climategate Controversy. Authors of the emails included climate scientists at a variety of institutions around the world. Focusing primarily on a few passages, contrarians claimed that the emails revealed that climate scientists had manipulated data to support the consensus position on anthropogenic climate change and had suppressed legitimate dissenting research in various ways e.
A number of independent investigations were subsequently conducted, all exonerating climate scientists of the charges of scientific fraud and misconduct that contrarians had alleged e.
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Some of the investigations, however, did find that climate scientists had failed to be sufficiently transparent, especially in their response to contrarian requests for station data used to estimate changes in global temperature ibid. The Hiatus Controversy. Global mean near-surface temperature increased significantly during the s but then showed little increase between the late s and the early s. By the mids, contrarians began to claim that global warming had stopped and that climate models and climate science were thus fundamentally flawed, since they had projected more warming.
Seasonality in long-term climate change
Part of the problem here was communication: graphs shared with policymakers and the public often highlighted the average of climate model projections, which smoothed out the significant variability seen in individual simulations and suggested a relatively steady warming; in fact, the observed rate of warming was not so different from that seen in some of the model projections see Schmidt ; Risbey et al.
A host of potential explanatory factors were identified—related to external forcing, internal variability, ocean heat uptake and errors in observational data—which contrarians portrayed as excuses. Subsequent investigation found evidence for actual contributions from most of the hypothesized factors, though discussion of their relative importance continues see Medhaug et al.
Contrarian dissent has impacted the practice of climate science in various ways. Most obviously, research is sometimes directed at least in part at rebutting contrarian claims and arguments. For example, a recent research paper related to tropospheric temperature trends Santer et al. Senate see also Lewandowsky et al.
In addition, Brysse et al. Drawing these threads together, Biddle and Leuschner suggest that contrarian dissent has impeded scientific progress in at least two ways:. They argue that, while dissent is science is often epistemically fruitful, the dissent expressed by climate contrarians has tended to be epistemically detrimental see also Biddle et al. The issue of anthropogenic climate change raises a host of challenging ethical questions. Most of these are beyond the scope of this entry on climate science.
A very brief discussion is provided here nevertheless, because the questions are important and because a full entry on the topic is not yet available. The basic ethical question is: What ought to be done about anthropogenic climate change, and by whom? The question arises because there is good evidence that climate change is already having harmful impacts on both humans and non-human nature, and because continued high rates of greenhouse gas emission can be expected to bring additional and more devastating harms in the future Field et al.
Attempting to address this basic ethical question, however, leads to further, complex questions of global and intergenerational justice, as well as to questions regarding our ethical obligations to non-human nature. Here are just a few examples: Do some nations, including those that have emitted large quantities of greenhouse gases in the past, have an obligation to bear more of the costs of climate change mitigation and adaptation than other nations?
See, e. When considering actions to mitigate climate change, how should the harms and benefits to future generations be weighed against those affecting people today?
How should impacts of climate change on non-human nature, including loss of biodiversity, be taken into account? Are there circumstances in which proposed geoengineering solutions—such as injecting sulfate aerosols into the stratosphere or seeding the oceans with carbon-absorbing phytoplankton—are ethically acceptable?
There is a large and growing philosophical literature engaging with these and related questions; some anthologies and book-length works include Arnold ; Broome ; Gardiner ; Gardiner et al. Climate Science First published Fri May 11, Introduction 2. Basic Concepts 3.
Climate Data 3. Climate Modeling 4. Anthropogenic Climate Change 5. Introduction The field of climate science emerged in the second half of the twentieth century. By contrast, a definition of climate change associated with a narrower, actualist view is: any systematic change in the long-term statistics of climate elements such as temperature, pressure, or winds sustained over several decades or longer. American Meteorological Society b The latter definition, unlike the former, allows that climate change might occur even in the absence of any changes in external conditions, as a result of natural processes internal to the climate system e.
Climate Data The sources and types of observational data employed in climate science are tremendously varied. Climate Modeling Models of the climate system, especially computer simulation models, have come to occupy a central place in both theoretical and applied research in climate science.
Thus, for example, the Intergovernmental Panel on Climate Change IPCC is able to report very high confidence that models reproduce the general features of the global-scale annual mean surface temperature increase over the historical period, including the more rapid warming in the second half of the 20 th century, and the cooling immediately following large volcanic eruptions.