Is there a scientific consensus on climate change?
The short answer is yes, in broad terms there is. That of course doesn’t mean that all scientists agree on everything having to do with climate change. First of, scientists are in general argumentative and stubborn: they will usually find something, be it a minor point, about which they disagree with someone else’s opinion or interpretation. But in light of that nature, the broad consensus about the recent climate chang is very strong indeed. This consensus is laid out in the IPCC reports (Intergovernmental Panel on Climate Change), by means of a thorough review of all recent scientific literature on the relevant topics. Minority opinions are also discussed in these reports, but are -as are all results- put in the context of the bigger picture of the literature as a whole. Indeed, this is how it should be done, for outsiders to get a proper view of what the current state of the science as a whole is.
What is the scientific consensus on climate change?
Short answer: Recent climate change is for a large part due to human activity.
- Surface temperature readings jiggle up and down, but over the course of the past century, their overall trend is up. Rising global temperature is also confirmed by the global retreat of glaciers, melting of ice sheets, poleward migration of species and increased ocean heat content. Issues with individual measurement stations do not impact this rising trend.
- The levels of carbon dioxide and other greenhouse gases in the atmosphere have also been rising. The concentration of CO2 in seawater has increased as well. The total observed increase is consistent with the amount of fuel humanity has burned and the amount of land it has cleared since the industrial revolution began. Isotope analysis confirms that the additional CO2 comes predominantly from fossil fuels.
- Basic theory (known since at least 150 years) links these two trends. Because of greenhouse gases, a smaller fraction of the thermal radiation emitted by the surface escapes into space. The planet becomes a net absorber of energy. The surface heats up and emits more thermal radiation, until the total amount of escaping thermal radiation again balances the energy deposited by sunlight. The radiative properties of CO2 have been measured over 150 years ago (Tyndall), and the warming of the earth resulting from our emissions was predicted shortly thereafter (Arrhenius). His prediction was quantitatively in the same ballpark as was later confirmed. Since then, research has -by and large- backed these results up into a mountain of evidence. Current global warming is a prediction come true. That doesn’t happen all that often in science…
- Natural factors, such as variations in the sun’s output, have been too small to account for the observed temperature increase.
Criticism on the consensus view
Most criticism concerns the first and the last points of the consensus view as stated above. The second and third points are hardly ever criticized, at least not by people minimally versed in physics. Indirectly however, they are often implied to be false.
- There has been a more or less constant attack on the observed temperature trend, even when it has now significantly risen above the level expected from natural variability of the weather. Fact is, the trend is so overwhelming and there are so many observations, even of different physical quantities, pointing in the same direction, that it really starts to be laughable to contest that the globe is warming. Known problems such as the urban heat island effect are corrected for in all temperature series, and the procedures to arrive at a global temperature are very carefully thought out and publicly documented. Of course climate scientists are all for good quality control of the observations, which admittedly still have a lot of issues to be dealt with (see eg here and here the latest “controversies” in how to deal with such issues). But when such criticisms are driven by an obvious desire to discredit the entire temperature trend (and, by extension, any mitigation policies) without proper justification for such far reaching claims, then I take the criticism with a grain of salt.
- Sometimes it is implicitly questioned that the current rise in CO2 is due to human activity, but in such a way that the claim is hard to recognize. E.g. by stating (correctly) that during the great ice ages the temperature rose before CO2 did, “skeptics” often conclude that the current CO2 rise is a consequence of warming rather than vice versa. But this last claim is patently false: we know for a fact that we put the CO2 in the air. (The cause-effect relation between CO2 and temperature goes in both directions; they can act as a feedback on each other. More CO2 causes warming; warming causes more CO2 emissions. It’s like the chicken and the egg.) As another example, the bogus documentary “The great global warming swindle” implicitly claims that the increased CO2 comes from the oceans (and thus, by extension, not from fossil fuel burning). This is so obviously wrong, it’s not even funny. (For a critique of this documentary, see e.g. here)
- The third point is implicitly pushed aside when trying to promote or exaggerate the sun’s role in climate change of the past century. As if by making the sun responsible for global warming the known physics of greenhouse gases magically stops functioning. Just like there is no knob to switch off gravity on the earth, there is no knob either to switch off the radiative effects of greenhouse gases. It’s there, deal with it. If it wasn’t there, the earth would be a whopping 30 degrees C colder than it is, and life as we know it would not exist. But what if the radiative effects of greenhouse gases are smaller than what is currently thought? While there is some playing room for the climate sensitivity (the equilibrium temperature response to a doubling in CO2 equivalents) to vary, this playing room is rather limited by several constraints from e.g. paleo-climate and climate response to large volcanic eruptions (see e.g. here and here). You would still have to explain away a whole body of observations in order to come up with a substantially larger contribution of the sun to current climate change than laid out in the IPCC reports.
- The sun’s energy output is huge, and variations in the sun have caused climate changes in the past. The sun’s energy output has not been as high as in the past century for thousands of years. True, but it only explains a small portion of the current warming, mainly in the early 20th century. Over the last 50 years the sun’s output has been roughly constant (decreasing weakly), so even with exotic and unsubstantiated magnifying mechanisms (e.g. cosmic rays, UV-dynamics feedbacks) the sun can not be held responsible for the recent global warming since 1975. Often you will hear the logical fallacy that “since the sun has been responsible for climate changes in the past, it must now also be responsible”. On an equal level are emotional arguments such as “How can we possibly influence something so big and complex such as the earth’s climate, when contrasting us little humans to that giant ball of energy in the sky?” Such arguments may sound plausible when listening to a radio-show; in a real scientific debate they are not taken seriously.