People ask "is it clear that human activity is directly responsible for climate change?" The context for answering this question must be another question: to what extent can the climate change all by itself?
The answer to the alternative question is: "a very great deal." Modern human beings appeared some time after about 50,000 years ago, and even then, anthropologists tell us that their numbers were very small until about 4000 years ago. Nonetheless, taking a cautious view, one might only examine climate change prior to 100,000 years ago.
Inferences about climate change before instruments and written records is the province primarily of geologists and geochemists. Their message is a very clear one: Earth has undergone enormous variations in climate state with changes taking place over times ranging from decades to millennia and longer.
Among the most extreme changes are the glacial-interglacial cycles in which, with the continents in their modern configuration dating back several million years, enormous ice caps waxed and waned over the Northern Hemisphere. Thus the UK, as well as all of western Europe, was under several kilometres of ice for thousands of years, interspersed with long intervals of a more benign climate such as that we have today.
These switches have taken place at intervals of between about 80,000 and 120,000 years for the last million years. Prior to that time, they appear to have occurred intermittently at about 40,000 year intervals. Even more dramatic changes took place in the deep past. It has been argued that during the Neogene period (about 24 to 1.8 million years ago), that the entire Earth froze over. Alternatively, over most of Earth's history, there were apparently no glaciers at all.
The glaciations are only the most dramatic of the inferred natural variability of the system.
Another problem concerns the counter-intuitive (for most people) behaviour of the consequences of random fluctuations in systems that have any kind of memory. As an example, consider the situation considered long ago by K. Hasselmann. The ocean is to be regarded as simply a completely passive reservoir of water with an initial temperature, T0. As such, its only physics we care about is its ability to store heat for very long periods (out to thousands of years in some instances).
Now we heat and cool the ocean over some small region using the atmosphere. To determine whether the ocean is to be heated and cooled on any given day, we simply flip a coin: if it's heads, we heat the ocean. If it's tails, we cool it by a like amount. Because we assume we have a true coin, the long-time average temperature of the ocean is the starting temperature, T0. Hasselmann pointed out, however, that the actual time history of temperature in this model ocean is very different from being near T0! Almost all the time, it is rather far from T0; in fact, the probability of its being T0 tends rapidly towards zero.
Most of the time, the ocean is either warm or cold compared to T0 and tends to stay that way for extended periods (we cannot predict whether it will be warm or cold, or the time interval over which it will stay warm or cold, but we can confidently predict the statistics of its departures from T0.
A consequence of this type of behaviour (and which a reader can easily check by having a small computer do the coin-tossing many times) is that systems with a memory of the past history of forcing can have very strange, unintuitive, behaviour that violates "common sense." The behaviour here can be understood by noting that if one tosses a true coin 2 million times, the probability of exactly 1 million heads and 1 million tails is very small. Instead, one expects a finite surplus of one or the other corresponding to excess heating or cooling.
We know that it is capable of remarkable changes without human intervention.
So now we come to the modern climate problem. We know that it is capable of remarkable changes without human intervention. We also know that it has elements with very long memory times (the ocean, the ice caps, and some land processes including the biota). There is the possibility of solar fluctuations about which we know very little. The instrumental record only goes back about 300 years (being very generous) and global coverage is only really available following World War II. In many cases, we have no direct evidence for the spatial structures of natural variations and so find it almost impossible to compare observed changes with those known not influenced by human activities.
Many scientists therefore rely upon numerical models of the climate system to calculate (1) the nature of natural variability with no human interference, and compare it to (2) the variability seen when human effects are included. This approach is a very sensible one, but the ability to test (calibrate) the models, which can be extraordinarily complex, for realism in both categories (1) and (2) is limited by the same observational data base already describe. At bottom, it is very difficult to determine the realism by which the models deal with either (1) or (2)
Thus at bottom, it is very difficult to separate human induced change from natural change, certainly not with the confidence we all seek. In these circumstances, it is essential to remember that the inability to prove human-induced change is not the same thing as a demonstration of its absence. It is probably true that most scientists would assign a very high probability that human-induced change is already strongly present in the climate system, while at the same time agreeing that clear-cut proof is not now available and may not be available for a long-time to come, if ever. Public policy has to be made on the basis of probabilities, not firm proof.
The same principle, however, does not apply when making decisions about war and peace. Then only absolute and final proof can justify a decision to invade another country.
Of course such proof can never be obtained until after an invasion.
No, it doesn't. Science may not be consensus, but at least we would need consensus on the probabilities, and we don't have it.
Why should we cripple and bankrupt ourselves over this hyped-up and misrepresented "calamity" when we almost totally ignore another possible global threat?
I'd like to see us spend a fraction of what Al Gore and his neo-Luddites would have us waste, on a system of detecting and dealing with the threat of large asteroids impacting Earth.
The probabilities involved in such a scenario are a lot easier to calculate, and the amelioration far less expensive for the desired results.
Actually, most scientists do not.
That sneaky phrase, "would assign a very high probability" is thoroughly false.
So that the rest of your paragraph is non-sensical.
But thank you for sharing your opinion.
This fiction has been challenged since at least 1995, when the following protest By one of the scientists in the then current IPCC report,
"In the early 1990s Lindzen was asked to contribute to the IPCC's 1995 report. At the time, he held (and still does) that untangling human influences from the natural variation of the global climate is next to impossible. When the report's summary came out, he was dismayed to read its conclusion: "The balance of evidence suggests that there is a discernible human influence on global climate."
"That struck me as bizarre," he says. "Because without saying how much the effect was, the statement had no meaning. If it was discernible and very small, for instance, it would be no problem."
Environmentalist Bill McKibbon referred to this phrase in an article in The Atlantic in May 1998: "The panel's 2,000 scientists, from every corner of the globe, summed up their findings in this dry but historic bit of understatement."
In an angry letter, Lindzen wrote that the full report "takes great pains to point out that the statement has no implications for the magnitude of the effect, is dependent on the [dubious] assumption that natural variability obtained from [computer] models is the same as that in nature, and, even with these caveats, is largely a subjective matter."