Wednesday 30 August 2017

The water-holding capacity of air increases 7 percent for every 1°C warming

This is a must-read and necessary contextualisation from Dahr Jamail.

He quotes Keith Trenberth who, I am proud to say is a New Zealander

Hurricane Harvey Shows What Climate Disruption-Amplified Flooding Can Do

Dahr Jamail

Residents navigate a flooded street that has been inundated with water from Hurricane Harvey on August 27, 2017 in Houston, Texas. Harvey, which made landfall north of Corpus Christi late Friday evening, is expected to dump upwards of 40 inches of rain in areas of Texas over the next couple of days. (Photo: Scott Olson / Getty Images)

29 August, 2017

My mother and father live just north of Houston. Here is the rather cryptic text message my mother, sent me late Sunday night:
Lost power. Got generator running, fridge on, light, running small AC in morning. Tired. Staying upstairs to escape generator noise.
Trees down. Wind up. Waiting for daylight to use chainsaws. Front entrance flooded.
We are okay. Tired.
Love you,

Tropical Storm Harvey, which made landfall near Corpus Christi last Friday as a Category 4 hurricane, has stalled over south-central Texas and has been dumping record levels of rain on this population-dense area. The area flooded in Texas, as of Sunday, was, staggeringly, the size of Lake Michigan. At the time of this writing, 450,000 Texans were expected to seek disaster aid.....

Sea-surface temperatures near Texas were between 2.7° and 7.2°F above average, making them some of the warmest ocean temperatures on Earth. This caused Harvey to ramp up from a tropical depression to a catastrophic Category 4 hurricane in merely two days' time.

"This is the main fuel for the storm," Kevin Trenberth, a senior scientist at the US National Center for Atmospheric Research told The Atlantic. "Although these storms occur naturally, the storm is apt to be more intense, maybe a bit bigger, longer-lasting, and with much heavier rainfalls [because of that ocean heat]."
Trenberth also told The Atlantic, "The human contribution can be up to 30 percent or so of the total rainfall coming out of the storm. It may have been a strong storm, and it may have caused a lot of problems anyway -- but [human-caused climate change] amplifies the damage considerably."
Trenberth is the author of a 2011 study titled, "Changes in precipitation with climate change," which shows how the water-holding capacity of air increases 7 percent for every 1°C warming, which naturally leads to an increase in the atmosphere's ability to hold water, and sets the conditions for epic rain events like Texas is experiencing today.

To read the article GO HERE

Changes in precipitation with climate change

Kevin E. Trenberth*

National Center for Atmospheric Research, Box 3000, Boulder, Colorado 80307, USA

ABSTRACT: There is a direct influence of global warming on precipitation. Increased heating leads to greater evaporation and thus surface drying, thereby increasing the intensity and duration of drought. However, the water holding capacity of air increases by about 7% per 1°C warming, which leads to increased water vapor in the atmosphere. Hence, storms, whether individual thunderstorms, extratropical rain or snow storms, or tropical cyclones, supplied with increased moisture, produce more intense precipitation events. Such events are observed to be widely occurring, even where total precipitation is decreasing: ‘it never rains but it pours!’ This increases the risk of flooding. The atmospheric and surface energy budget plays a critical role in the hydrological cycle, and also in the slower rate of change that occurs in total precipitation than total column water vapor. With modest changes in winds, patterns of precipitation do not change much, but result in dry areas becoming drier (generally throughout the subtropics) and wet areas becoming wetter, especially in the mid- to high latitudes: the ‘rich get richer and the poor get poorer’. This pattern is simulated by climate models and is projected to continue into the future. Because, with warming, more precipitation occurs as rain instead of snow and snow melts earlier, there is increased runoff and risk of flooding in early spring, but increased risk of drought in summer, especially over continental areas. However, with more precipitation per unit of upward motion in the atmosphere, i.e. ‘more bang for the buck’, atmospheric circulation weakens, causing monsoons to falter. In the tropics and subtropics, precipitation patterns are dominated by shifts as sea surface temperatures change, with El Niño a good example. The volcanic eruption of Mount Pinatubo in 1991 led to an unprecedented drop in land precipitation and runoff, and to widespread drought, as precipitation shifted from land to oceans and evaporation faltered, providing lessons for possible geoengineering. Most models simulate precipitation that occurs prematurely and too often, and with insufficient intensity, resulting in recycling that is too large and a lifetime of moisture in the atmosphere that is too short, which affects runoff and soil moisture.

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