What we know.

Somebody on a Facebook thread said something silly about climate change. Somebody else said "I know a climate scientist, let's see what he says." That scientist was me.

I responded at perhaps greater length than the occasion deserved, so I have decided to preserve it here. Here's my first pass at "what we know".

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Here's what I am absolutely (not 97% or 99%) sure of:

We understand how the atmosphere works pretty darn well. See how the hurricanes are going almost exactly where the models predicted? That's an amazing achievement, and there's a lot of science behind it.

To understand the weather at the surface of a solid planet, one of the keys is the composition of the atmosphere, particularly of gases and suspended particles that absorb radiation. That radiation can be either from the sun (mostly visible and ultraviolet) or thermal radiation from the surface (infrared). Over the long term, the energy coming in matches the energy going out. So the composition of the atmosphere sets the surface conditions. If it's harder for the infrared to get out, the planet is warmer than otherwise. This is not just about the Earth, it's universal.

We call gases that absorb infrared "greenhouse gases", and in understanding the complicated climate history of the Earth, carbon dioxide is one of the two most important of these, the other being water vapor.

Carbon flows in closed cycles between the soil, the plants, the atmosphere and the ocean. Natural changes in CO2 concentration, while very important, are relatively slow. Humans recently have been digging up carbon at a prodigious rate, and burning it to create CO2, altering the amount of carbon in the system. This has been accelerating with economic activity, and has recently become large enough that it matters to the climate. About half the extra CO2 stays in the atmosphere, warming and changing the climate at rates that are much faster than natural systems evolved for nor than human systems are prepared for. Despite increasingly loud and consistent (yes, consistent) warnings from scientists going back as far as the 1950s, the problem continues to accelerate.

Extra carbon in the system does not go away for a very long time (unless we undertake very expensive measures to slowly draw it down artificially). This means that with each passing day the burden we leave to our descendants gets worse.

The main sources of damage are expected to include ecological stress, sea level rise, and direct heat stress. Patterns of drought and flood are likely to change, and unexpected events will take specific places by surprise.

Also, the extra carbon alters ocean chemistry (ocean acidification) which directly impacts shellfish and plankton and upsets the ocean food chain. This is just beginning to be noticeable and is going to get much worse.

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People say "97% of scientists" agree with this, but in my experience it is an underestimate. I don't personally know of any competent scientist who disagrees with the above. The "3%" seem to be outsiders who don't know their stuff.

Consider this editorial in the Houston Chronicle, which had a similar purpose to this posting. It asserts "to the best of our knowledge, there are no climate scientists in Texas who disagree with the mainstream view of climate science." As far as I know, nobody stepped up to contest that claim. Texas is a big state not known for reticence. The naysayer contingent in Texas is zero per cent.


There's a great deal more that we know, but that's basically the core of what we are sure of about climate change. There's a lot we suspect but aren't sure of, mostly about the consequences of all this and what to do about it. There's much debate about all that, but most people who think about it seriously are very concerned to say the least.

UPDATE: Kerry Emanuel also tried to answer this question, for an audience of engineers, in somewhat greater detail. Video is here: https://www.youtube.com/watch?v=7so8GRCWA1k