Saturday, December 30, 2017

How We Know Climate Change Is Real

The recent short term cold snap in the Midwest and Northeast US caused Donald Trump, a/k/a Der Trumpenführer, a/k/a the idiot-in-chief, to tweet mocking the science behind climate change and touting his decision to pull out of a global deal to combat planetary warming.   The stupidity and ignorance of the tweet prompted the Weather Channel to shoot back and explain the difference between weather (short term events) and climate (long term phenomenon), and reconfirm that that 2017 is still projected to be the warmest year on record and that, in fact, the eastern cold snap Trump referenced in his tweet was actually evidence of a warming climate.  Sadly, Trump and his ignorance embracing evangelical base despise science and knowledge that counter their strongly held prejudices or, in the case of evangelicals, fantasy world.  A column in the New York Times looks at how we know climate change is real and that it is increasing severe climate events.  Here are highlights:
This was a year of devastating weather, including historic hurricanes and wildfires here in the United States. Did climate change play a role? Increasingly, scientists are able to answer that question — and increasingly, the answer is yes. 
Consider Hurricane Harvey, which caused enormous destruction along the Gulf Coast; it will cost an estimated $180 billion to recover from the hurricane’s storm surge, high winds and record-setting precipitation and flooding. Did global warming contribute to this disaster?
The word “contribute” is key. This doesn’t mean that without global warming, there wouldn’t have been a hurricane. Rather, the question is whether changes in the climate raised the odds of producing extreme conditions.
It is therefore critical to examine all of the contributing factors. In the case of Hurricane Harvey, these include the warm ocean that provided energy for the storm; the elevated sea level on top of which the storm surge occurred; the atmospheric pressure pattern that contributed to the storm’s stalling over the coast; and the atmospheric water vapor that provided moisture for the record-setting precipitation.
 The first step is to ask whether historical changes have been observed in any of the factors. For example, ocean temperatures have increased in recent decades. Applying the same statistical techniques used in engineering, medicine and finance, we can analyze whether those increases have changed the odds of achieving this year’s warm temperatures in the tropical Atlantic and Gulf of Mexico.
 Based on previous warm years, we can expect to find that human-generated warming influenced this year’s ocean temperatures.  We also know that global warming is increasing the moisture in the atmosphere, meaning that a given storm can produce more precipitation.
 Further, Hurricane Harvey’s stalling over the coast was critical for the record rainfall. The exact meteorological causes are complex, but the pattern of atmospheric pressure across North America played an important role. We have found that global warming increased the odds of the pressure pattern that contributed to the 2010 Russian heat wave that killed more than 50,000 people. We can likewise look back at pressure patterns during past hurricane seasons and examine whether global warming has altered the odds of patterns similar to Hurricane Harvey’s.
In addition to the heavy rainfall, storm surge contributed to coastal flooding. When hurricanes make landfall, low pressure and strong winds push water onto land. By increasing the mean sea level, global warming has “raised the floor” from which storm surge occurs. As a result, a storm is more likely to cause extensive flooding. Sea-level rise tripled the odds of Hurricane Sandy’s flood level in 2012. A similar analysis can be applied to the Hurricane Harvey storm surge.
Our scientific framework can also be applied to other events. Like Harvey’s devastation, California’s ravaging wildfires arose from a confluence of factors. Strong, dry winds were the most immediate contributor. In addition, the protracted drought that killed millions of trees created substantial fuel. After the drought, an extremely wet winter was followed by severely hot, dry conditions in the summer and fall, which together produced near-record fuel for fires. Although each of these specific factors will need to be analyzed, we already know that global warming has increased fuel aridity in the West, meaning that fires are more likely to encounter large amounts of dry fuel.
There is now ample evidence that global warming has influenced extremes in the United States and around the world through such factors as temperature, atmospheric moisture and sea level. This doesn’t mean that every event has a human fingerprint. But it does mean that we can expect more years like this one, when our old expectations no longer apply.

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