Why do we think that the observed increased concentrations of CO2 and Methane will warm the earth?
1) Basic physics
2) Water vapour feedbacks from recent measurement of radiative outflow from satellites & Models integrating these observations
3) Observations of the climate warming up already (see below for detailed refs)
4) Observations CO2 of the ice ages (showing evidence for positive feedback as well as a very close link between temperature and CO2 and Methane)
Concentrations of CO2
Concentrations of CO2 went between 180 (ice age) and 280ppm (warm period between ice age). They are now at 388ppm: higher than the last few million years; the sun is also getting stronger over the very long term.
Science of Greenhouse Effect
Greenhouse gases increase the flow of energy into the Earth. It has been estimated that a concentration of CO2 of 550 parts per million (before industrialization the level was 275 parts per million) would leave to 3.7 Watts extra heat imput per square metre of the Earth's surface area.
The Stefan Boltzmann law would shows that the heat radiated from the earth's surface increases by about 3.2 Watts per square metre per degree Celsius rise in temperature. Therefore, the Earth's temperature would need to rise by about 1.2 degrees Celsius to balance out this rise in temperature.
However, we know that warmer air has a higher absolute level of humidity (in otherwords it contains more water vapour). Water vapour is also a greenhouse gas, and so this traps heat too.
We can estimate that water gives a positive feedack of -1.6 Watts per square metre per degree Celsius rise in temperature.
This should be compared to 'StefanBoltzmann' extra heat flow of 3.2W/m2K, giving net effect of 1.6W/m2K
When we include this effect (but assume no other feedbacks), that means that the earth would have to rise in temperature by 2.3 Celsius (not 1.2 Celsius) before the outflow of heat balanced the extra inflow.
So CO2 drives temperature, that increases humidity, and that leads to the water vapour feedback, which can be observed. See this article.
All the evidence is put together with computer models, but we don't really need computer models to estimate these issues, we can work it out ourselves from science and observations
Evidence of warming
- Science of Climate Change
- Stratosphere is cooling, a typical indicator of climate change
- Tropopause is rising, as expected
- "Tropical hotspot signature" I don't know if this is actually expected or observed
- See Taylor for an understanding of these issues
Very many different observations around the world e.g. temperature measurements, rate of glacier melt, species shifts, Artic sea ice, sea surface temperatures, coral reef bleaching, heat waves:
- Sea level rise
- Storms, Hurricanes
- Droughts and Floods, and other impacts
- Sea Ice (ice at the north pole is declining rapidly, at the south pole increasing slightly)
- Coral reefs (some evidence of decline, especially bleaching in a large proportion of populations)
- Polar Bears ("of the 19 subpopulations of polar bears, eight are declining, three are stable, one is increasing, and seven have insufficient data")
Most of these show some evidence of climate change. People will I'm sure, come to their own conclusions.
There are some arguments about climate change by self-styled 'sceptics'. Here is an explanation of the more complex issues.
Uncertainty & Risk?
Of course, there is always discussion and debate, but the fact that there are big risks shouldn't blind us to doing something to secure ourselves against those risks.
We know that the earth responds to a lag to our behaviours. We already have seen serious effects to climate change (see 'evidence of warming' elsewhere in this reply) and the rate of increase of greenhouse gas concentrations is itself accelerating (think of putting the foot down when you see a road traffic accident). Don't you think it might be good to be a little bit safe rather than sorry?
We need a much stronger treaty that doesn't only include global targets, but also coordinated taxes.
It has been estimated that the investment required to decarbonize the UK is around £600bn (which would spent mostly on UK resources). The UK consumes 1.7million barrels of oil per day or 620 million barrels per year, with a value (at $80/bbl) of $50billion (£30billion).
We use 91.1 billion cubic metres of gas per year present, worth £11billion (at 35p per therm or 13p/cu m). So we spend more than £40bn per year on fossil fuels; replacing this with renewable and nuclear infrastructure could get a return on our investment of 15 years. Not bad.
Good, strong, climate policies could increase investment in real infrastructure, providing jobs, and making us less dependent on foreign oil! Hurrah!