Since 2007, every report from the UN IPCC has concluded that warming of the climate system is “unequivocal”. Thermometers show warming, including thermometers far from cities (so it is not just an urban-heat-island effect), thermometers in the ground, in oceans, on balloons, and looking down from satellites. Most of the world’s ice is shrinking, including in places getting more snowfall. Most of the changes in where different things live, and when they do things during a year, are moving in the direction expected with warming. Models forced with the known natural causes match changes in the late 1800s and early 1900s, but not since. Adding human forcings gives a very good match to what happened all the way along. This match includes not merely global-mean surface temperature, but also many aspects of the “fingerprint” — regional temperature changes, vertical temperature changes, oceanic temperature changes, etc. Note that the whole forcing must be included; particles from smokestacks do the volcano job of blocking the sun, but don’t stay up very long, whereas greenhouse gases warm the climate and stay up longer. (The cooling after World War II was forced by human-produced aerosols, based on available information. And the idea that scientists were warning about global cooling in the 1970s, so beloved of the “skeptics”, is a misrepresentation. Newsweek ran an article on this, and some interesting science was being done on ice-age cycling, and on cooling by particulates, and the possibility of a “nuclear winter”, but the scientific community was already primarily focused on warming at that time, and never released any consensus documents pointing to cooling. And while Newsweek may be a respected general-information source, it is NOT a respected scientific source.)
Suppose, for a moment, that you believe the sun has caused the recent warming. There is no support for this in the data; almost 30 years worth of satellite data show no trend in solar output, or a very slight drop, while temperatures on Earth were going up. But suppose you believe that the satellite data are wrong, that the sun has been getting brighter, and that the temperature changes on the planet are solar-caused. A clear prediction of this solar model is warming in the stratosphere as well as in the troposphere, as more energy is added to both. But a greenhouse-gas hypothesis points to tropospheric warming coupled to stratospheric cooling, as the greenhouse gases hold the energy closer to the surface and radiate from high elevation. So, what do the data show? Tropospheric warming-and stratospheric cooling. The “fingerprint” is human, not solar.
The future looks warmer, unless we change our behavior. Everywhere and everywhen we look, more carbon dioxide makes it warmer. This is a fundamental result of physics — there is no serious suggestion that this could be wrong, and extraordinarily strong evidence that it is right. The data agree; warmth and high carbon dioxide have gone together, the warmth is explainable through the known effects of carbon dioxide, and the warmth is not explainable if the effects of carbon dioxide are omitted. When carbon dioxide has been fairly constant, small effects from sun, volcanoes, and perhaps other things have been evident, but these have acted more as fine-tuning knobs than as coarse adjustments.
The planet’s climate is stabilized strongly by the black-body radiative feedback over very short times, and by the feedbacks involving rock weathering and carbon dioxide over very long times. Between, the feedbacks are largely amplifying.
The biggest amplifier is linked to water vapor. At higher temperature, the saturation vapor pressure is higher, and the “kinetics” (evaporation if dry air is over water) are faster. Over the ocean (which is most of the planet), relative humidity is more-or-less constant (the wind mixes dry air down from above into the wet air below, so the air holds most but not all of the water for saturation), and warmer places thus have more water vapor. Warming is increasing water vapor. And water vapor is a powerful greenhouse gas. Humans cannot change water vapor very much directly — the residence time is barely over a week, so the water vapor we put up comes down quickly — but by changing the temperature through other greenhouse gases, we can change atmospheric water vapor because there is an immense ocean out there to respond to the warming by putting more water in the air.
The ice-albedo feedback is straightforward. With warming, snow and ice melt, and that increases absorption of sunlight in the Earth system, warming the planet. Vegetative feedbacks also can matter — we may have cooled the planet a bit by replacing dark forests by more-reflective croplands — but vegetative feedbacks can’t be really huge (they are limited to land, and that particular trees-to-crops switch is limited to croplands). Clouds bring the biggest uncertainties, but the main circulation pattern of the Earth is highly stable, hence the upward and downward motions of air fairly well fixed, hence one cannot make immense changes in cloud easily.
Comparing various models indicates that, if we start from a stable climate similar to that of the Holocene, and then double carbon dioxide with no other forcings, and let water vapor, snow, cloud, etc. respond, the planet will average about 3 C warmer. The direct effect of the carbon dioxide is just over 1 C with the rest from feedbacks. The uncertainty is usually given as 1.5 C, although increasingly it appears that the lower end of that (warming from doubled carbon dioxide being less than 2 C) is more likely to be wishful thinking than science. Efforts to match the history of the last century, of the ice-age cycling, and of longer times, generally agree with the models, strongly reject lower values, but typically include a small possibility of a larger or much larger sensitivity (so things could be a little better than the central estimate, a little worse, or much worse). With enough fossil fuels still in the ground that we could quadruple atmospheric carbon dioxide, and perhaps octuple, and with each doubling of atmospheric carbon dioxide having a roughly equivalent effect on temperature, a central estimate of warming in a burn-it-all future may approach 10 C or more than 15 F; if we burn it all, and the climate is really insensitive, we may get only half of that warming, or we may get twice that much. (And, remember that the difference between the ice-age world and the recent one was about 10 F. Note that we won’t get the full equilibrium warming, because the ocean takes a while to warm up, but carbon dioxide stays up long enough that we are likely to get most of the equilibrium warming.)
The “so what” part of this takes a lot more discussion, which won’t all fit here. In general, warmer temperatures are likely to bring less winter, more summer, sea-level rise, more droughts and more floods (fewer precipitation events with more water in them), drying in grain belts in summer, potential spread of tropical diseases, loss of ecosystems and species. Initially, there is not likely to be too much economic impact in the cold places where vigorous economies are driving the change, but negative impacts in the warm places where great numbers of people live. Eventually, harm is expected to spread to almost everyone almost everywhere.
Economic analysis of these issues is much cruder than physical-scientific analysis (in part because the uncertainties in the physical science are magnified in the economics). Typically, analyses show that an optimal response (considering only the economy, and not ethical issues) involves at least some investment to reduce greenhouse-gas emissions now. A complete fix is often priced at around 1% of the world economy, after a few decades of serious effort.
Of course, this is science, not revealed truth, and is subject to errors. The distribution of possible outcomes is “interesting” — things could be a little better than sketched here, or a little worse, or a lot worse. North Atlantic shutdowns, hundred-year droughts, ice-sheet collapses, and climate sensitivity of 4 or 5 C rather than 3 C for doubled carbon dioxide are clearly within the range of outcomes consistent with current knowledge, whereas no- change or tiny-change worlds are not.