“Climate change skepticism" and "climate change denial" refer to denial, dismissal or unwarranted doubt of the scientific consensus on the rate and extent of global warming, its significance, or its connection to human behavior, in whole or in part." Wikipedia
There is doubt and then there's acknowledgment of uncertainty. So if someone says: “we don’t know how well or how quickly ecosystems or species will adapt to climate change”, that’s not doubting the possibility of catastrophe – it’s just saying we don’t know for sure what’s going to happen.
There is doubt and then there's acknowledgment that our knowledge is limited. Climate change is complicated. The models are still being worked out. A lot of guesswork is involved: factors affecting the efficiency, scalability, reliability, cost, and adoption of renewable sources of energy; rate and geography of population growth; agricultural productivity and resilience; pace of economic development around the globe; changes in patterns of consumption and production. How much will developing countries depend on fossil fuels as they urbanize and become prosperous? Must development go hand-in-hand with increasing fossil fuel use – or might that linkage be broken by technology transfer and the increasing availability of cheap green energy?
There is a general scientific consensus that anthropogenic climate change is happening. There is not a consensus on the rate, magnitude or impact of climate change. The Intergovernmental Panel on Climate Change (IPCC) has presented four plausible trajectories of GHG concentrations in the upper atmosphere, on the basis of which they estimate how much the climate will warm over various time periods, along with the potential impact on planet, biosphere, and human society. These are hypothetical effects, expressed with varying degrees of confidence.
One can “doubt” a consensus, but what would it mean to doubt a range of estimates about how much and how fast the climate will warm? IPCC’s lowest estimate for global warming this century is 0.3 to 1.7 °C. If we go with Wikipedia’s definition of climate change denier at the beginning of this post, doubting that global warming would reach .3 °C by 2100 would probably qualify one as a denier. However, I am not familiar of any cases where individuals are accused of being climate change deniers because their °C estimate of global warming by 2100 doesn't reach the IPCC threshold. Such exactitude is not really part of the labeling game.
More likely someone will be called a denier if he/she doesn't seem all that concerned about climate change or thinks its effects will be minor and manageable. These individuals may very well endorse anthropogenic climate change and accept that global warming is proceeding at a pace consistent with at least the lower range of IPCC projections. They may still be labeled climate change deniers because they don’t appear to appreciate the “significance” of climate change - which is, after all, part of the Wikipedia definition.
Next we'll meet some public figures who have been described as deniers. We'll consider the loci of their doubt or denial: the existence, rate, extent, or significance of climate change. - in whole or part.