We've known for nearly 200 years that the Earth ought to be much colder than it is, given only its distance from the Sun,
and that therefore the atmosphere must restrict the passage of infrared radiation (heat) to space, thus causing the surface
to remain warm (Joseph Fourier, 1820s).
We've known for 150 years that CO2 is one of the principal gases responsible for this 'greenhouse effect' (Tyndall, 1859).
We've known for over 100 years that doubling atmospheric CO2 would raise global temperature by several degrees Celsius
In 1981 Dr. James Hansen of NASA published 'Climate impact of increasing atmospheric carbon dioxide' and projected the
anthropogenic (man-made) global warming we would expect to see under various scenarios in the growth of fossil fuel use,
from fast growth through slow growth to no growth at all -
Since then, global fossil fuel use has seen fast growth as countries such as China and India have undergone rapid industrialisation.
We can see how well Dr. Hansen's projections of global warming under this scenario have matched reality so far:
Observed global temperatures from 1950 to 2010 are shown in blue, and the red curve is a trendline fitted to the
observations and projected into the future. Here is Dr. Hansen's projection of the 'slow growth' scenario just for the
period to 2020, compared to observations:
It's very clear that anthropogenic global warming is running at or above the highest projection in Dr. Hansen's paper,
far outside the range of any natural variability, and that continued unrestrained CO2 emissions during this century
will commit us to global warming of several degrees C, and a climate not seen since the age of the dinosaurs. The
consequences of this for a human population of 7 billion and more will be disastrous. Modern human civilisation has
developed in the relatively benign and unusually stable climate of the last 10,000 years, and will not cope well with a
dramatically altered world of desertification, rising sea level, devastated agriculture, decimated ocean life and increasing
extreme weather events. We can expect to see increasing famine, mass migration as large areas of cropland become
unsuitable for cultivation, increasing resource wars, catastrophic water shortages, food crises even in the richest countries,
and an increasing numbers of failed states which descend into chaos and pose an increasing security risk to other
countries. The military of many countries now see global warming as the greatest security threat of this century. Global
warming is real, it's due to human activity, and Dr. Hansen and others have been right about it for many decades. It's
about time we started paying attention.
Hansen's 1988 Projections
We can also look at Hansen's global warming projections as presented to the US Congress in 1988:
Of the three emissions scenarios (A, B and C), scenario B (green curve) is closest to the actual emissions to date.
We can see that the output of Hansen's climate model was a little too warm compared to subsequent observations
(the black curve). The model was run with a climate sensitivity of 4°C. Subsequent work by many researchers has
shown that the most likely value is 3°C. When recalculated with this updated value for climate sensitivity, Hansen's
model run for scenario B is remarkably close to actual observations: