A friend, Chase Saunders, a man I have known since he was in the third grade when he and my third grade son became good friends, asked me about pan evaporation and why it was decreasing. Was it due to a reduction in the sun's irradiance? I did some quick research to find what the heck pan evaporation was. Basically you take a tank of water, put it outdoors and fill it up with water to a specified level. Then you measure how much water evaporates each day. And every day when you measure the evaporation, you re-fill it. If it rains, you take that into account and then remove water from the pan to bring it back to the appropriate level. Such measurements help farmers know how much irrigation they need to do to keep the crops alive. Pan evaporation measurements have been carried out for decades around the globe.
Now, one would expect that since hot water evaporates more quickly than cold water, that as the earth warms, the evaporation would increase. After all, if you put a pan on the stove and keep the water at 150 deg F it will evaporate faster than the room temperature pan kept over by the sink. So, why is it that pan evaporation over the globe has dropped over the past several decades?
The oldest article on this that I could find was from the third Nature magazine I have in my library. The following chart is published there.
You can see that the evaporation rate has been going down since 1945 yet the world has been warming--so the thermometers say. So is there other data showing the same thing? Yes, Australia shows the same thing.
Japan also shows a decrease in pan evaporation.
And all but one region of the US.
" There were no sites showing statistically significant increases in pan evaporation. A rough indicative average for the decline in the pan evaporation rate across all 19 sites was about 2 mm a^−2 and was generally consistent with the previously reported declines of 2–4 mm a^−2 from the Northern Hemisphere and from Australia." MICHAEL L. RODERICK and GRAHAM D. FARQUHAR ," CHANGES IN NEW ZEALAND PAN EVAPORATION SINCE THE 1970s," Int. J. Climatol. 25: 2031–2039 (2005)
The -2 mm per year drop in evaporation reported above is not unusual.
This article, Michael L. Roderick, Michael T. Hobbins and Graham D. Farquhar "Pan Evaporation Trends and the Terrestrial Water Balance. I. Principles and Observations," Geography Compass 3/2 (2009): 746–760, has other declines in pan evaporation rates.
India -12 mm/year
China -2.8 to 3.9 mm/year
Thailand -10 mm/year
Turkey -24 mm/year
Canada -1 mm/year
Israel, Ireland, Kuwait, and the UK are the only places with increasing pan evaporation rates, but the UK and Ireland also have some studies showing declines. This decline is a widespread phenomenon and is contradictory to the expectations of global warming.
Roderick and Farquhar published an article in Science suggesting that these results could be explained by a 3% per decade drop in solar irradiance. Needless to say they admit that they have encountered scepticism over this claim. Their article can be found for non-subscribers to Science at this place.
The IPCC rejects any significant changes in solar irradiance.
The radiative forcing due to changes in solar irradiance for the period since 1750 is estimated to be about +0.3 Wm-2, most of which occurred during the first half of the 20th century. Since the late 1970s, satellite instruments have observed small oscillations due to the 11-year solar cycle. Mechanisms for the amplification of solar effects on climate have been proposed, but currently lack a rigorous theoretical or observational basis.
Nasa data shows no decline in solar irradiance over the past 35 years.
So, what other alternatives exist? Humidity is one. If the air is more humid it will slow evaporation. The problem is that this is ruled out for the following reason:
"However, this explanation for decreasing pan
evaporation is unsatisfactory for two reasons.
First, it only predicts changes in pan evaporation
in water-limited environments. The
problem is that some areas are not waterlimited,
and in wet environments the evaporation
from pans and the surrounding environment
have both declined.Michael L. Roderick and Graham D. Farquhar, "The Cause of Decreased Pan Evaporation over the Past 50 Years" Science, 298(2002), p. 1410
So, if humidity and a change in solar irradiance are ruled out as explanations for why it is harder to evaporate a pan of water today than 50 years ago, are there any other possibilities?
Yes, it is obvious from the pictures that Anthony Watts at this site that there are huge problems with the thermometer record. Also if you just take the raw data, it shows half a degree less of warming than the final edited GISS data does. Editing adds heat. The bias of the climatologists take in raw data showing a slight warming trend, and then turn out a final record that is warming by 1/2 deg C more. But that warming trend in the original data is probably mostly from the urban heat island effect.
The incompetence of the climatologists at measuring temperatures can be seen in any of my analyses of the temperature records between two closely spaced towns. Just look through my blog's archive to find them.
Most of the pan evaporation stations are in rural areas and the information is used to aid in irrigation decisions.
The thermometer record is often in cities where the temperatures are affected by urban heat and air conditioner exhaust fans. Maybe the explanation for the pan paradox is that the earth ISN'T warming but is really cooling. That too would explain why it is harder to evaporate a pan today than 50 years ago, contrary to the expectations of global warming. It would also fit into the general decline of temperature since the last deglaciation, as seen in the deuterium in the ice cores.
The earth has been cooling for 10,000 years. Maybe it still is.