How do we know that global warming is ongoing? How do we know that it is caused by human activities?
The lines of evidence are numerous, and like a house which is supported by many pillars, the entire edifice is stable even if one or several might fail. This has not happened- the letter writer is spreading disinformation (whether he knows it or not) on a deadly serious subject. Satellite measurements starting in the 1970's have documented a drop in infrared radiation emitted from the Earth's surface out into space, along wavelengths absorbed only by carbon dioxide. Wavelengths absorbed by water vapor have remained relatively constant. This evidence shows that the enhanced greenhouse warming observed is due to excess carbon dioxide. Analysis of the carbon atoms in our atmosphere, compared to those in samples from before the Industrial Revolution, show that the isotopes (carbon atoms varying in the amount of neutrons in their nucleus) come from fossil fuels.
In 1940, for some time onward, the bulk of fossil fuel burning was "dirty" coal. This is the largest reason for the lack of observed mid-century warming. Burning coal not only releases CO2, but also sulfur dioxide. Tiny sulfur dioxide particles in the atmosphere cause cooling by reflecting light back into space.
One way people cloud the facts of this debate is by throwing in short term cooling trends. Climate is defined as weather averaged over time, the shortest time period for climate being 30 years, due to natural cycles such as the Solar Cycle and El Nino. 1998 was unusually warm due to a strong El Nino. Recently, the sun has reached the minimum energy of its cycle. Both of these together have caused slight cooling since 1998. However, the sun is on the upswing, and the lack of a strong El Nino means that the oceans have been absorbing heat, where it is out of sight, but not out of mind. Its effects will be felt in the future- prepare yourselves.
In response to this.