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2018-08-29

Har det blivit vanligare med extremt väder?

Det sägs ofta att ökad global värme gör det vanliga med extremt väder. Dag för faktakoll.

IPCC gjorde i sin senaste stora rapport AR5 en noggran undersökning av extremvädret. Här är slutsatserna

1. “Current datasets indicate no significant observed trends in global tropical cyclone frequency over the past century”
2. “No robust trends in annual numbers of tropical storms, hurricanes and major hurricanes counts have been identified over the past 100 years”
3. “In summary, there continues to be a lack of evidence and thus low confidence regarding the sign of trend in the magnitude and/or frequency of floods on a global scale”
4. “The statement about the absence of trends in impacts attributable to natural or anthropogenic climate change holds for tropical and extratropical storms and tornados”
5. “In summary, there is low confidence in observed trends in small-scale severe weather phenomena such as hail and thunderstorms”
6. “In summary, the current assessment concludes that there is not enough evidence at present to suggest more than low confidence in a global-scale observed trend in drought or dryness (lack of rainfall) since the middle of the 20th century.”
7. “In summary, confidence in large scale changes in the intensity of extreme extratropical cyclones since 1900 is low”.

Sedan dess har det kommit åtskilliga vetenskapliga rapporter som gett olika resultat. En speciellt intressant iaktagelse finns i ‘”Constrained work output of the moist atmospheric heat engine in a warming climate” publicerad i Science. Författarna menar att intensifieringen av vattnets kretslopp minskar kraften till atmosfärisk cirkulation och till intensiva stormar. (expense of its ability to do work, such as powering large-scale atmospheric circulation or fueling more very intense storms.)  
http://science.sciencemag.org/content/347/6221/540

Weakening of the global atmospheric circulation with global warming, Joakim Kjellsson 2015

This study finds that the global atmospheric circulation weakens, i.e. transports less mass, by about 5 % per degree of global warming as a result of anthropogenic emissions. This is found by calculating a global thermodynamic mass stream function and diagnosing long-term changes in a set of climate-model simulations in the Intergovernmental Panel on Climate Change 5th assessment report. The global atmospheric circulation also expands in thermodynamic coordinates due to increased dry static energy and latent heat in the tropical troposphere. The surface warming and moistening follows a Clausius–Clapeyron relation and tropical moist ascent remains nearly moist adiabatic. Furthermore, the meridional overturning circulation is found to only weaken slightly suggesting that the dominant change is in the zonally asymmetric circulation, i.e. the Walker circulation. The total poleward energy transport increases while the poleward mass transport decreases. The results are found to be robust across the climate models studied.

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Taggar: Extrem väder IPCC AR5

Red: Hans Iwan Bratt. epost: hibratt@gmail.com.