There is overwhelming scientific evidence that the majority of the rapid warming of our planet over the past century has been forced by increasing greenhouse gas concentrations. The concentration of carbon dioxide—the most important long-lived greenhouse gas—is now greater than it has been for at least 800,000 years, and if global economic growth continues and nothing is done to curtail emissions, its level at the end of this century will reach values not seen since the Eocene period, 50 million years ago. Pushing the climate system this hard and this fast entails serious risks to human civilization, engendered in rising sea levels and associated incidence of storm-related coastal flooding, decreasing habitability of tropical and arid regions, increasing acidification of ocean waters and associated risks to marine ecosystems, and destabilization of the hydrologic cycle with attendant increases in food and water shortages. The latter is especially worrying because of the propensity for past fluctuations in food and water supplies to drive civilizational collapse, rapid migrations, and armed conflict.
While climate science is increasingly confident in its attribution of recent climate change to human-caused changes in greenhouse gases and aerosols, the innate skepticism of scientists leads to large uncertainty in climate projections, with possible outcomes ranging from the benign to the catastrophic. There is no scientific justification for the confidence expressed by some that climate change entails little or no risk.
There is some basis for optimism that civilization can greatly reduce climate risk by incentivizing development of carbon-free energy sources and technology for extracting CO2 from the atmosphere and/or directly from emissions sources. Renewables can power 20%–60% of current demand, or more if better energy-storage technology is invented. Next generation nuclear fission has many advantages over 1960s nuclear technology and, once developed, can be ramped up to meet a large fraction of demand in 15 years, judging from the experience of countries like France and Sweden. (Nuclear-energy costs over the lifetime of power plants are competitive with coal and oil.) There is also renewed optimism that nuclear fusion, a basically limitless clean source of energy, may become commercially viable in 20 to 30 years. Unfortunately, this will be too late to significantly curtail major climate risk, but it does provide an ultimate target for clean-energy production.
At the present rate of consumption, oil and gas reserves are projected to be exhausted by late in this century, and coal early in the next. Thus in the not-too-distant future fossil fuels will have to be replaced anyway; to mitigate climate risk that transition would need to be advanced by several decades. Other countries, notably China, are investing in advanced carbon-free energy sources, including nuclear fission. Those nations and/or businesses that develop carbon-free energy early and well will gain an important competitive advantage in what is currently a $6 trillion energy market.