Warmer and wetter. But not as wet as I thought.

The Global Mean Temperature Anomaly compared to the average during the years 1951 to 1980. The data shows that 2014 was the hottest year on record. A link to the data source is shown at the end of this post.

The Global Mean Temperature Anomaly compared to the average during the years 1951 to 1980. The data shows that 2014 was the hottest year on record. A link to the data source is shown at the end of this post.

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.

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.

Schematic illustration of the way molecules in a liquid evaporate.

A close-up view of the situation near a liquid surface. Molecules in the liquid jiggle and occasionally  molecules near the surface gain enough energy to escape i.e. evaporate, to form a vapour (gas) in the space above the liquid surface. At high temperatures, the molecular jiggling is more violent and the rate at which molecules leave the surface increases.

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.

If air flow over a bowl constantly removes evaporated molecules, then the concentration of vapour never builds up. In this case, the rate of evaporation increases at approximately 6% for every 1 ºC rise in temperature.

If air flow over a bowl constantly removes evaporated molecules, then the concentration of vapour never builds up. In this case, the rate of evaporation increases at approximately 6% for every 1 degree 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.

Air flow over a beaker constantly removes evaporated molecules near the mouth of the beaker. But lower down inside the beaker the air is less disturbed, and the amount of vapour in the air is in balance with rate at which molecules leave the liquid surface.

Air flow over a beaker constantly removes evaporated molecules near the mouth of the beaker. But lower down inside the beaker the air is less disturbed, and the amount of vapour in the air is in balance with rate at which molecules leave the liquid 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.

The situation at the ocean surface. There is a thin boundary layer just a few millimetres in extent, in which the amount of water vapour is almost in balance with the rate of evaporation from the liquid surface.

The situation at the ocean surface. There is a thin boundary layer just a few millimetres in extent, in which the amount of water vapour is almost in balance with the rate of evaporation from the liquid surface.

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.

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2 Responses to “Warmer and wetter. But not as wet as I thought.”

  1. iamamro Says:

    Interesting. This comes as a surprise to me. Of all things one’s asked to explain to the lay person, climate change is the number one (when one is cornered as the one scientist at an event).

  2. telescoper Says:

    Reblogged this on In the Dark and commented:
    I reblogged the original article, so I should also reblog the correction and clarifications…

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