Forecasts of Climate Change Predict Our Lack of Rain?

December 9, 2017

Illustration (Image credit: Wix)

 

People are speaking to me about the weather, which is a lot better than when they only glared.  Instead of being angry with me, they want to know what's with all the changes, and why it's stopped raining.

 

But, first the weather: our storm has departed and it will be warming up quite nicely as warm air flows move in with a trough of lower pressure centered over the deserts to our southeast.  The warmth will peak on Tuesday, prior to the first night of Hanukkah.  

 

A cold front should then arrive Tuesday night from the west and temperatures should fall through Friday morning. In contrast to earlier forecasts, we're now getting indications of a chance of rain from Thursday through Shabbat. At the moment, it doesn't look like a heavy rain.

 

Jerusalem forecast (click for other forecasts):

Image credit: The Jerusalem Herald
 
Getting back to "why" it has stopped raining: regional climate change models forecast that increased carbon dioxide levels should lead to more frequent, if not severe droughts in our area, with the drought more intense in northern areas.  This would be in response to a northward drift of the winter jet stream that until recently frequently meandered in and out of the eastern Mediterranean. When the jet stream moves southward from Europe, cold air associated with southward bends (or troughs) causes storms to develop, which intensify near Cyprus (called "Cyprus Lows"). These are our main rain producers.


There are those who object to the idea that climate change caused by greenhouses gases (produced by power plants, in industrial processes, agriculture, transportation, etc.) is a serious problem or if it exists at all. The Intergovernmental Panel on Climate Change (IPCC) has relied on a range of climate forecast models to warn us of dire consequences in the decades ahead, producing a range of possible temperature increases during this century from devastatingly hot to more temperate, with the projected changes depending on how much greenhouse gases we produce.  

 

In reality, the global mean temperatures have increased by a few tenths of a degree, but this is below or just within the lower boundary of the predictions. In other words, more than 95% of the global forecasts are hotter than our current reality.  Clearly, there are physical processes not well represented by most of the models.
 
On the other hand, there have been serious fires in the western US, where it was terribly hot during the summer, and winter rains are late. It just snowed in Houston, Texas; Atlanta, Georgia just had its largest snow in years at several inches or more -- and it is only early December.
 
How do we explain that?  In simple terms, the increase in greenhouse gases has increased the energy "retained" by the atmosphere.  More energy in the system means that the waves that grow because of latitudinal (north/south or south/north) temperature differences can amplify more strongly (deeply) than they would without the added energy. These waves can transport unseasonably hot weather further north than "normal," and colder air further south.  On the net the earth is warmer, but some areas experience unseasonable heat or cold.
 
As for us in Israel, we're ahead of last year in terms of rainfall.  In fact, last year at this time it had barely begun to rain.  After rainfall began on the 13th of December, it rained a number of times before the end of the month. January was very dry and February not particularly wet. We had about 50% of the normal seasonal rainfall. Last year's fall was particularly dusty and warm, while this year's fall has felt more winter-like.  Based on this, I think there is hope for this year.

 

Happy Hanukkah!

 

Dr. Lynn is a lecturer at The Hebrew University of Jerusalem, Earth Sciences Department.  He is also CEO of Weather It Is, LTD, a company that specializes in reducing weather risk.
 

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בס"ד

...הָרִימִי בַכֹּחַ קוֹלֵךְ מְבַשֶּׂרֶת יְרוּשָׁלִָם הָרִימִי אַל תִּירָאִי אִמְרִי לְעָרֵי יְהוּדָה הִנֵּה אֱלֹקֵיכֶם! (ישעיה  מ:ט)

...Raise your voice with strength, herald of Jerusalem; raise it, do not be afraid; say to the cities of Judah, "Here is your G-d!"

(Isaiah 40:9)

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