Who'll Stop The Rain?

Guest Post by Willis Eschenbach

I haven’t posted here at WUWT in a bit. I’ve been preoccupied writing for my own blog, Skating Under The Ice.  It’s a work in progress. However, my climate work continues. There’s a paper out about a year old, unfortunately paywalled, regarding precipitation with some interesting conclusions. [UPDATE: available here, h/t to commenter Taphonomic]. The paper is called Changes in annual precipitation over the Earth’s land mass excluding Antarctica from the 18th century to 2013, by W.A. van Wijngaarden and A. Syed. Here is what ScienceDirect lists as the key points of the paper:

Highlights

• Over 1½ million monthly precipitation totals observed at 1000 stations in 114 countries analysed.

• Data record much longer than 3 recent conflicting studies that analysed a few decades of data.

• No substantial difference found for stations located at northern, tropical and southern latitudes.

• No substantial difference found for stations experiencing dry, moderate and wet climates.

• No significant global precipitation change from 1850 to present.

The Abstract of the paper reads:

Precipitation measurements made at nearly 1000 stations located in 114 countries were studied. Each station had at least 100 years of observations resulting in a dataset comprising over 1½ million monthly precipitation amounts. Data for some stations extend back to the 1700s although most of the data exist for the period after 1850. The total annual precipitation was found if all monthly data in a given year were present. 

The percentage annual precipitation change relative to 1961-90 was plotted for 6 continents; as well as for stations at different latitudes and those experiencing low, moderate and high annual precipitation totals. The trends for precipitation change together with their 95% confidence intervals were found for various periods of time. Most trends exhibited no clear precipitation change. 

The global changes in precipitation over the Earth’s land mass excluding Antarctica relative to 1961-90 were estimated to be: -1.2 ± 1.7, 2.6 ± 2.5 and -5.4 ± 8.1% per century for the periods 1850-2000, 1900-2000 and 1950-2000, respectively. A change of 1% per century corresponds to a precipitation change of 0.09 mm/year.

Being a visual kind of guy, I converted the one sigma errors to 95% confidence intervals, and graphed up the trends:

As you can see, there is no statistically significant change in global rainfall, whether we look back fifty, a hundred, or a hundred and fifty years.

How are we to understand this? Well, to start with it is another example of the amazing stability of the global climate system.

However, I have to ask myself, given that in general a warmer world is a wetter world .. why so little change? This is often a valuable exercise, trying to understand a negative result.

One thing that comes to mind is that the effect may be there, but not be big enough to see. After all, the land has warmed up something like one degree Celsius or so since 1850, from something like 14°C to 15°C.

So … IF we assume that atmospheric water goes up linearly with increasing absolute temperature T, the temperature has gone up from about 287 Kelvin to 288 Kelvin, which is about a third of a percent … so we wouldn’t expect to see that.

On the other hand, IF water vapor goes up by T^4, then the increase is a bit larger. But it’s still only a percent and a half, which still would not be visible.

Finally, IF atmosphere water goes up from approximately zero at 0°C to 100% at 100°C … well, even then it’s only one degree, so that would still only be 1%.

So one possibility is that warmer is wetter, but the temperature change over the last century and a half is not large enough to cause an observable change in rainfall.

Another possibility is more subtle. As I write this, the wind outside is pushing the rain sideways at something like 20° from the vertical. This means that my rain gauge shown below, complete with a chambered nautilus shell in the window that we picked up on some beach “long ago and in another country”, that gauge is not recording the actual rainfall amount.

So … how much is the error? Well, at the simplest level it’s equal to the cosine of the angle of the vertical. If the rain is falling at 45° the gauge will read about 30% less than the true value. At 20° from the vertical like today, it only reads about 6% low. However, in reality of course it’s much more complicated. For example, for a given wind, smaller droplets are more likely to be blown away from the gauge. So light rains will have a greater error than heavy rains.

The total analysis is quite complex, depending on the exact shape of the rain gauge and how it is affected by the winds. There’s a fascinating analysis here.

Setting the details aside, it is clear that a slow secular change in either global average wind speed or global average raindrop size could easily make a 1% change in the long-term rainfall records …

In any case, it seems that the rumored droughts and floods due to rising global temperature are not visible in either the global or regional analyses, nor in the short, medium, or long-term analyses, nor in the tropic, subtropic, or polar analyses, nor in the dry, medium or wet locations. Here is their final conclusion:

Stations experiencing low, moderate and heavy annual precipitation did not show very different precipitation trends. This indicates deserts/jungles are neither expanding nor shrinking due to changes in precipitation patterns. It is therefore reasonable to conclude that some caution is warranted about claiming that large changes to global precipitation have occurred during the last 150 years.

Another alarmist myth run aground on an inconvenient reef of facts …

Best to all,

w.

PLEASE: In your comments, be so kind as to QUOTE THE EXACT WORDS YOU ARE REFERRING TO. This prevents misunderstandings regarding your subject.

REGARDING THE POST TITLE:

WHO’LL STOP THE RAIN—Creedence Clearwater Revival Band

Long as I remember the rain been comin’ down

Clouds of mystery pourin’ confusion on the ground.

Good men through the ages tryin’ to find the sun.

And I wonder, still I wonder, who’ll stop the rain.

I went down Virginia seekin’ shelter from the storm

Caught up in the fable I watched the tower grow

Five year plans and new deals wrapped in golden chains.

And I wonder, still I wonder, who’ll stop the rain.

Heard the singers playin’, how we cheered for more.

The crowd had rushed together tryin’ to keep warm.

Still the rain kept pourin’, fallin’ on my ears

And I wonder, still I wonder, who’ll stop the rain.