Last week I wrote about how 2014 was warmest year in UK history.
This week two US labs have confirmed that 2014 also appeared to be the warmest year on Earth for many generations.
- NASA’s Goddard Institute for Space Studies (GISS) who have the data if you want to plot it yourself.
- The National Oceanic and Atmospheric Administration (NOAA) who have an interesting discussion about trends.
And looking at the graph above, it’s not hard to see why people – myself included – are alarmed by the trend. Hiatus? What Hiatus?
But on last week’s blog I also got something wrong. I said that
Roughly speaking, every 1 ºC causes a 6% increase in the rate of evaporation – which gives rise to an additional 6% of global rainfall.
Actually the amount of additional rainfall isn’t known – we can’t measure it well enough – but it is probably about 2% ± 1% for each 1 ºC rise in temperature. Here the ± 1% indicates the uncertainty in the estimate.
- You can get the best estimate from Chapter 7 of the 5th Assessment Report of the Intergovernmental Panel on Climate Change. Chapter 7 concerns Clouds and Aerosols and Section 7.6.2 concerns The Effects of Global Warming on Large-Scale Precipitation Trends. You can get the report as a pdf document from this web link.
The reason I was wrong was because I had completely misunderstood something.
At this point I recommend you look away and wait for my next – much more interesting – article. But if you want to know why I was wrong, read on.
Liquids and Vapours
Liquids evaporate. This occurs because the jiggling molecules near the surface of the liquid occasionally get enough energy to escape the electrical attraction of their neighbours and fly off into the space above the liquid surface and form what we call a vapour.
If the molecules leave for ever – for instance if a wind blows over an open bowl of water, then the liquid keeps evaporating. For water the rate at which molecules leave the liquid surface increase by 6% for each 1 ºC rise in temperature.
And this is how I imagined the situation at the surface of the ocean. I thought molecules would evaporate and be immediately carried way into the atmosphere. If that were true, then the only way to maintain balance and to stop the oceans constantly evaporating is to increase precipitation by 6%.
But in fact there is another way for molecules to get back to the liquid. If the molecules that evaporate are not immediately blown away, then their concentration above the liquid surface increases. Eventually the concentration of vapour increases until as many molecules re-enter the liquid surface as leave it.
If we imagine now not a bowl but a beaker, then the concentration of water vapour just above the surface of the liquid is almost in balance with the liquid. The rate at which the wind removes water vapour from the beaker depends on the details of the way the wind creates eddy currents in the mouth of the beaker.
Further down in the beaker – the air is less disturbed – and just above the surface of the liquid there is a balance between the number of molecules leaving the liquid surface and those re-entering the surface.
Surprisingly – to me at least – for water at the surface of Earth’s oceans, the situation is more like the ‘beaker’ case than the ‘open bowl’ case. Immediately above the ocean surface is a layer of air – called the boundary layer – in which the amount of water vapour is almost in balance with the ocean.
When the temperature of the ocean surface changes by 1 ºC, the rate at which molecules enter the boundary layer from the ocean surface increase by 6%. But because of this boundary layer, many molecules which evaporated from the ocean surface return directly to the ocean – without having to journey into the atmosphere first.
The rate at which global precipitation increases depends on the rate at which the vapour in the boundary layer is removed and transported into the atmosphere. And this process is complicated – depending on many details of the way the winds blow across the ocean.
At least that’s what I think now.