The Spectrum of Scientific Opinion

The Consensus

Disagreement is part of the business of science. Whenever scientists get together they find themselves discussing the latest results and more likely than not they will part ways disagreeing on whether the results are trustworthy, or if they are trustworthy whether they are important, or if important what they imply for existing consensus, and what needs to be done next. On the other hand, there are things scientists agree on, and these opinions are called the consensus. It is valuable to know what the consensus is, but it is also true that consensus is still no more than an opinion, albeit an opinion shared by the great majority of knowledgeable people.

It is incorrect to assume that opinion is unsuited as a basis for action. In the world of finances, people invest money based on opinions about the chances for success. In matters of war and peace, governments invest in military infrastructure and weapon systems based on opinions concerning the attitudes and intentions of other nations. Of course, the opinions of venture capitalists and defense experts will normally be well founded. Entrepreneurs will have access to facts regarding the value of a new product and the competency of the chief executive officer in charge of bringing it to market. Statesmen will know about the state of armament of an adversary and the foreign policy moves. Some opinions are held very strongly. In finances, it would be that the US dollar will not be suddenly worthless. In military affairs, it would be that superior intelligence is just as important as superior weapons if one is to prevail in a conflict.

Likewise, in science there are strongly held opinions. For the topic of climate change they are as follows:

The greenhouse gases in the Earth's atmosphere are rising rapidly as a result of man-made emissions.
Emission of trace gases such as carbon dioxide, methane, nitrous oxide and CFC’s increases the greenhouse effect and produces global warming.
A doubling of carbon dioxide results in an average increase of roughly 2.5°C, with a range of uncertainty between 1°C and 5°C.
Global warming has in fact occurred for more than 100 years. Some (unknown) proportion of the warming is due to human activities.
The relative importance of different kinds of ways that gases get emitted has changed through time. For carbon dioxide it was forestation, then burning of coal, and today the burning of petroleum and natural gas.
Close to one half of the additional greenhouse effect comes from emissions other than carbon dioxide.
Today's CO₂ values in the atmosphere are about 30 percent over background and are unprecedented for the last 400,000 years. Thus, there is no analog situation available to us from which to deduce what normally happens at the present greenhouse gas concentrations.
Besides greenhouse gases (including water vapor), many other factors must be considered in calculating the radiation balance, including particles in the atmosphere (natural and man-made) and changes in albedo from changing ground cover and cloud cover, and we do not understand all these processes very well.
In addition to the increase in greenhouse gases, the changing output of energy from the Sun as well as effects from volcanic eruptions must be included in any assessment of global warming.
Given the complexity of the problem, we need computer models, which summarize and organize our physical knowledge. They are a lot better than guessing or “back-of-the-envelope” calculations in predicting possible outcomes of an increased greenhouse effect.

Some less strongly held opinions, but still worth taking into account:

The last two decades are entirely unusual as far as climate. For instance, we now see the melting of high-elevation glaciers in the tropics that have previously persisted for more than 10,000 years.
There is no guarantee that the ocean will continue to take up carbon dioxide at the accustomed rate in the future and may well decrease its uptake.
There is no guarantee that the ocean will take up heat in the accustomed manner in the future.
If the production of oceanic deep water is diminished between Greenland and Norway, there will be less heat going into northern Europe, leading to strong climate changes there.
If the trade winds lose strength, as a result of a diminished temperature difference between high and low latitudes (as expected) coastal upwelling will decrease and so will the productivity of the ocean. This will be hard on coastal fisheries.
There is no guarantee that large portions of the western Antarctic ice shelf will remain stable in a warming climate, especially as the sea level rises substantially. Shelf ice can be lifted off from where it is grounded, and will then drift out to sea. This removes back-pressure from the land ice behind it, which can then flow more freely. Glaciers which start to flow more quickly warm up from internal friction, and become unstable. The sea level rise corresponding to these processes could be as large as several meters.
There is no evidence that a modest warming and a modest increase in carbon dioxide is harmful to the ecosystems of the world, although continued change to a sufficiently high level will certainly produce changes in the biota.

Instructors’ Opinions

Some opinions of your instructors (who are working scientists in the field of climate history):

Uncertainties regarding the human influence on global warming have been vastly overplayed by special interests (including governments). The climate is warming, and according to all we know about physics and climate change, it is supposed to get warmer .
Human activities in their impact on natural systems including climate are much more powerful than generally realized, even among scientists. The reason is that textbooks change more slowly than the climate.
The great success of physics and engineering in changing our lives has made us less sensitive to the limitations posed by the environment. People in cities and scientists in laboratories tend to forget that frost, flood and drought are outside of our control.
It is risky to seriously mess with systems that you do not understand and cannot control.