1) Sceptics or denialists are a single homogenous group
This is clearly wrong, but this does not stop anyone who is sceptical in any way being lumped into this group and being attacked continuously. For the record the following are just some types of sceptisism:
- Those who fully accept dangerous global warming but think that an ETS system is a bad policy (e.g. James Hansen of NASA);
- Those who accept that the world has warmed and will continue to warm in a serious way due to human contributions, but think there are a combination of human causes, not just CO2 (e.g. Prof. Roger Pielke Sr);
- Those who accept that the world has warmed and will continue to warm significantly, primarily due to CO2 emissions, but don't accept the predictions of catastrophe (e.g. Tom Fuller);
- Those who accept the world has warmed to some degree, partly due to human influence, but don't accept that human influence is dominant or likely to lead to dangerous warming in the future;
- Those who do not accept there has been any warming in the last 50-100 years and deny that humans can influence the climate at all.
2) Sceptics are sceptical of climate change
This is a language trick. Very few people are sceptical of the fact that the world's climate is changing. The sceptisism is directed at the extent and causes of changes to climate. Natural variations in climate have always occured. So this misconception may be convenient shorthand, but it is inaccurate and misleading. The sceptisism is most often directed at human CO2 emissions being the most significant driver of dangerous global warming.
3) The evidence for CO2 driven dangerous climate change is overwhelming
This is obviously contentious. There is certainly overwhelming evdience that CO2 operates as a greenhouse gas. If you fill one glass container with air, and another with air plus extra CO2, then shine a light on both containers, the one with extra CO2 will get warmer. This is not where most sceptics find problems. The real problems that sceptics have include the following (courtesy of climate-skeptic.com):
- Claim A: Nearly every scientist, skeptic and alarmist alike, agree that the first order warming from CO2 is small. Catastrophic forecasts that demand immediate government action are based on a second theory that the climate temperature system is dominated by positive feedback. There is little understanding of these feedbacks, at least in their net effect, and no basis for assuming feedbacks in a long-term stable system are strongly net positive. As a note, the claim is that the net feedbacks are not positive, so demonstration of single one-off positive feedbacks, like ice albedo, are not sufficient to disprove this claim. In particular, the role of the water cycle and cloud formation are very much in dispute.
- Claim B: At no point have climate scientists ever reconciled the claims of the dendroclimatologists like Michael Mann that world temperatures were incredibly stable for thousands of years before man burned fossil fuels with the claim that the climate system is driven by very high net positive feedbacks. There is nothing in the feedback assumptions that applies uniquely to CO2 forcing, so these feedbacks, if they exist today, should have existed in the past and almost certainly have made temperatures highly variable, if not unstable.
- Claim C: On its face, the climate model assumptions (including high positive feedbacks) of substantial warming from small changes in CO2 are inconsistent with relatively modest past warming. Scientists use what is essentially an arbitrary plug variable to handle this, assuming anthropogenic aerosols have historically masked what would be higher past warming levels. The arbitrariness of the plug is obvious given that most models include a cooling effect of aerosols in direct proportion to their warming effect from CO2, two phenomenon that should not be linked in nature, but are linked if modelers are trying to force their climate models to balance. Further, since aerosols are short lived and only cover about 10% of the globe’s surface in any volume, nearly heroic levels of cooling effects must be assumed, since it takes 10C of cooling from the 10% area of effect to get 1C cooling in the global averages.
- Claim D: The key issue is the effect of CO2 vs. other effects in the complex climate system. We know CO2 causes some warming in a lab, but how much on the real earth? The main evidence climate scientists have is that their climate models are unable to replicate the warming from 1975-1998 without the use of man-made CO2 — in other words, they claim their models are unable to replicate the warming with natural factors alone. But these models are not anywhere near good enough to be relied on for this conclusion, particularly since they admittedly leave out any number of natural factors, such as ocean cycles and longer term cycles like the one that drove the little ice age, and admit to not understanding many others, such as cloud formation.
- Claim E: There are multiple alternate explanations for the 1975-1998 warming other than manmade CO2. All likely contributed (along with CO2) but it there is no evidence to give most of the blame to Co2. Other factors include ocean cycles (this corresponded to a PDO warm phase), the sun (this corresponded to the most intense period of the sun in the last 100 years), mankind’s land use changes (driving both urban heating effects as well as rural changes with alterations in land use), and a continuing recovery from the Little Ice Age, perhaps the coldest period in the last 5000 years.
- Claim F: Climate scientists claim that the .4-.5C warming from 1975-1998 cannot have been caused natural variations. This has never been reconciled with the fact that the 0.6C warming from 1910 to 1940 was almost certainly due mostly to natural forces. Also, the claim that natural forcings could not have caused a 0.2C per decade warming in the 80’s and 90’s cannot be reconciled with the the current claimed natural “masking” of anthropogenic warming that must be on the order of 0.2C per decade.
- Claim G: Climate scientists are embarrassing themselves in the use of the word “climate change.” First, the only mechanism ever expressed for CO2 to change climate is via warming. If there is no warming, then CO2 can’t be causing climate change by any mechanism anyone has ever suggested. So saying that “climate change is accelerating” (just Google it) when warming has stopped is disingenuous, and a false marketing effort to try to keep the alarm ringing. Second, the attempts by scientists who should know better to identify weather events at the tails of the normal distribution and claim that these are evidence of a shift in the mean of the distribution is ridiculous. There are no long term US trends in droughts or wet weather, nor in global cyclonic activity, nor in US tornadoes. But every drought, hurricane, flood, or tornado is cited as evidence of accelerating climate change (see my ppt slide deck for the data). This is absurd.
I haven't read enough of the peer-reviewed literature to give an exhaustive summary, but suffice to say despite efforts of certain scientists to exclude sceptical papers from publication, some have still slipped through the cracks. A brief list of more recent articles follows:
- Christy, et. al., Surface temperature variations in East Africa and possible causes, Journal of Climate (2009)
- Lindzen and Choi, On the determination of climate feedbacks from ERBE data,Geophysical Research Letters (2009)
- Christy, et. al., A comparison of tropical temperature trends with model predictions International Journal of Climatology (2007)
- Anastasios, et. al., A new dynamical mechanism for major climate shifts Geophysical Research Letters (2007)
- Loehle, A 2000-year global temperature reconstruction based on non-treering proxies Energy & Environment (2007)