Climate Change is slow on the scale of a human lifetime, and small in comparison with day-to-day and season-to-season variability. A change of average summer temperatures by 1 °C over 10 years is small compared to a day-to-day variability of typically ±5 °C in the course of a typical summer.
So we rely on meteorologist, and climatologists and statisticians to tell us when ‘something is happening’. However many people are understandably sceptical of the opinion of ‘experts’. In many areas – for example in the field of human nutrition – expert opinion is fickle and changeable. So at what point can we expect to be able to make up our own minds using our own senses?
That is the question addressed by James Hansen in a paper (Perception of climate change: pdf: open access) published by the National Academy of Science (of the United States of America).The paper considers extremes of weather and in particular, hot summer events in the Northern Hemisphere. I am not sure that the paper completely answers the question it sets out to answer, but nonetheless is it is very readable.
The conclusion of the paper is shocking. Compared with a reference period from 1950 to 1980, extremely hot summers in the Northern Hemisphere land area are now more than ten times as common. In a typical recent summer, these anomalous conditions now exist over 10% of the Northern Hemisphere land area, compared with a few tenths of a percent in the reference period.
Hansen concludes that these events (e.g. the current US drought , the Russian heatwave in 2010) are now so commonplace that they can they can definitely be said to be due to a changed climate. The extreme summer events arise not just from the warming trend, but also an increase in climate variability – essential extreme weather – compared to the locally-expected average weather. NASA have prepared a nice animation here.
Hansen is clearly convinced, and he convinces me. But other aspects of the changed climate conspire against the perception of change. For example if cold dry winters are replaced by not-so-cold but snowy winters, then people perceive those as being ‘worse’, even if the actual temperatures are higher.
Hansen is addressing this issue for a reason. It is not until the majority of people in a democracy become personally convinced that the Climate really is changing, that it will become inevitable that politicians will do something. Reducing greenhouse emissions – mainly carbon dioxide – is not fun for anyone. But if the US loses a significant fraction of its corn crop for more than a year or two in a row, then even the denialists and contrarians may think again.