Global warming is an environmental crisis because it threatens to destabilize our climate. The climate has changed in the past, and human actions and emissions very probably speed up the the process of change.
The climate is very important to a vast array of economic activities. Most importantly, it threatens agriculture ( and thus food production) world wide. Rain patterns might change, ecological boundaries shift introducing new species in agricultural landscapes. It is not exactly known what the effects might be. Even the IPCC predictions remain rough and very diverse across the planet. But a vast unsupervised experiment with our climate through fossil fuel burning might be rather stupid.
Which brings me to the second challenge. Fossil fuel is the driver of our modern economy. We are hooked on it, although renewables have eased off our ever growing appetite a little. Unlike solving other environmental problems, which usually can be solved by placing a filter or finding a substitute chemical, global warming means reforming our entire economy. We need entire new forms of capturing and storing energy, if future societies are going to look anywhere near the same as ours now. If it is not because of our climate ( which is reason enough, i reckon) its because we are running out. fossils are finite.
A third crisis is that of the mismatch between science and society. Our economies have evolved to a global scale, with everything affecting everything at some part of the globe. Understanding the environmental impacts of such a finely meshed and wide reaching system requires the best knowledge seeking vehicle we have: science. gather evidence, pose hypothesis, check hypothesis with new data, reform hypothesis and so on. It is a highly specialized task, which not only requires a deep understanding of the scientific method, but also a lot of knowledge of the system at hand. Because the system is so difficult, developing knowledge on a little part of the system takes entire careers. The general public will feel displaced, their everyday knowledge often does not match the highly specialized conjectures of professional scientists.
The debate in the scientific, which requires a fundamental doubt about every conjecture and hypothesis, is seen by the public as proof that even the scientific world cannot shield off uncertainty. Science is discredited as an effective tool for developing methods to deal with future changes. Sometimes, lay people will simply deny the possibility that the future can bring anything different than it has in the past.
We have but one earth, and we are experimenting with it on a vast scale. Every corner of the globe, our presence has taken its toll, whether its PCB's on the poles, a sea the size of france made of plastic soup, or a CO2 concentration that has risen by 30 % in the last century. Science might be a poor tool to predict the effects of our actions. But we have no better tool, and this tool tells us that we have used up our room for experimenting.
