1. The earth has been warming since the 1600s, when we experienced what is known as the Little Ice Age, and has warmed about 0.85ºC since the mid-19th Century. Temperatures today are similar to those in what is known at the Medieval Warm Period, as shown in the following temperature reconstruction graph.
|Reconstructed global temperature past 2,000 years (Loehe and UKMO data)|
2. Carbon dioxide is a greenhouse gas. It makes the Earth habitable for life, and life would not exist on Earth if there was no CO2 in the atmosphere. An increase in atmospheric CO2, all other things being equal, would be expected to lead to an increase in average global temperatures but with a diminishing effect (the physics behind this is explained in the "Into the Laboratory" section of this article).
3. Mankind's carbon emissions, mostly generated through the burning of fossil fuels, contribute to the CO2 in the atmosphere. The exact extent of mankind's contribution to the increase in CO2 is unknown because we don't know the net natural contribution, but in recent years mankind's total emissions has been roughly equal to the increase in CO2 so many scientists just assume that human emissions account for all of the increase. If this was true then CO2 levels would have been constant prior to the development of human civilisation, which is patently not true as the following graph shows.
|Reconstructed atmospheric CO2 levels (100PPM) from various sources|
4. Carbon dioxide is NOT a pollutant. To claim that is to say that all life on Earth pollutes the environment merely by living, which is patently ridiculous. The concentration of CO2 in the atmosphere has increased from around 250ppm to 400ppm since the 18th Century but current CO2 levels are not dangerous. In fact, we are still only a little above the minimum levels of atmospheric CO2 necessary to sustain life on Earth and scientists now accept that rising CO2 levels have led to increased greening of the world, including a net increase in rainforest and a receding Sahara Desert.
5. The current increase in global temperature levels are NOT dangerous to life. Human civilisation flourished in warm periods and geographical areas (such as the Middle East and Mediterranean) rather than in cooler periods and climes precisely because less resources needed to be spent creating shelter and growing food in warmer areas, leaving more resources to be devoted to civilisational advancement.
6. Dangerous weather events are NOT increasing around the world. In fact, the last decade has seen fewer hurricanes and storms than any other decade since modern records began. Total deaths attributed to all extreme weather events globally declined by more than 90% since the 1920s, in spite of a four-fold rise in population and much more complete reporting of such events (source: Goklany). Many more people die each year from extreme cold than from heat and therefore an increase in global temperatures is likely to further lower climate deaths. Millions die in the third world every year from toxic heating fuels such as dung and biomass, deaths that would be prevented if they converted to natural gas or other clean fossil fuels.
So what is the point of the international political consensus on reducing carbon dioxide emissions? We know that human carbon dioxide emissions won't lead to runaway global warming and that further CO2 increases won't be harmful to life. On the other hand, we can say with certainty that policies that reduce the ability of people in cooler climates to heat their houses will lead to more deaths, and likewise any policies that reduce the ability for people in the third world to shift to less toxic energy sources than they currently use.
I accept, as do almost all scientists, that human activity contributes to changes in the climate, but I think the evidence does not support the proposition that mankind's carbon emissions are the dominant factor in recent increases in global average temperatures. But what about the argument that prudent risk management means we should cut our emissions anyway? Well, a prudent risk management strategy always considers the costs of mitigation and at the moment the costs of mitigation far outweigh the costs of the risk. Eliminating human carbon emissions to stop global warming is akin to amputating your leg to get rid of a muscle ache.
This is why the Paris Accord is bad policy and why I hope Trump' sticks to his guns and withdraws from it.