In my opinion, much of the confusion and public uncertainty over climate change is a lack of understanding of the difference between weather and climate.

  • Weather consists of the short term patterns of temperature, pressure, winds and precipitation from hour to hour, day to day, year to year.
  • Climate is defined as the 30 year average of weather patterns.
If you imagine standing at the water's edge on a beach, weather is like the movement of the water backwards and forwards as the waves break and recede, but climate is like the tide - the longer term trend of the water in or out. You can easily see the former, but the latter is harder to notice until your feet get wet.

The reason why the difference between the two is so crucial, is that man made carbon emissions have weak effect on weather. Despite an increased concentration of greenhouse gases, winter will always be colder than its preceding summer, and indeed each year has a reasonable possibility of being cooler than the previous one. If you have a look at the average global temperatures on the graph below (the blue points) and zero in on any 3-4 years, it is hard to see a pattern. In fact on this scale, the biggest influence on the temperature in any one year is the El Nino/La Nina weather system in the Pacific. For example 2008 was a strong La Nina year. La Nina brings temperature down which is why 2008 was the coolest year since 2000.

Climate change deniers and sceptics will be rubbing their hands with glee if they've read the last paragraph, but what they don't understand is that man-made carbon emissions may have a weak effect day to day or even year to year, but it is a persistent effect, slowly ratcheting up average temperatures. This is because carbon emissions, unlike say water vapour, must be taken out of the atmosphere by chemical or biological transformations (eg photosynthesis). 2008 may have been a rather cold year relative to the previous 6, but it is still much warmer than any year before 1998. If you look at the red line (a 5 year rolling average, so still quite short term in the sense of climate) there is a clear upward trend. This trend cannot be explained by natural cycles, but it can be explained by increased carbon emissions.

So if anyone ever says to you "How can there be global warming if we had two weeks snow in February?", your standard reply should be "That's just weather!"