Sunday, April 24, 2016

Has Climate Change Really Improved U.S. Weather? - by Kevin Trenberth

According to a new report published in Nature on April 20, 2016 by Patrick Egan and Megan Mullin, weather conditions have “improved” for the vast majority of Americans over the past 40 years.  This, they argue, explains why there has been little public demand so far for a policy response to climate change.
However, when we consider what Americans “prefer” with respect to weather, it is important to consider all variations in the weather – across hours, days and especially the extremes – rather than simply looking at annual averages.

After all, no one experiences long-term average weather, but we do increasingly experience weather extremes and their impacts on our health, safety and well-being.
Impacts of climate extremes
... For temperature, it is fluctuations up and down around averages that draw attention and impact lives.  Increasing heat waves, intensifying droughts, and expanding wildfires are taking a ruinous toll, especially in summer months.

The wildfire season is many weeks longer than it used to be.  Wildfires are local, but they affect us all through smoke and air quality, insurance, and fire-fighting costs.  Increasing pollen, allergies, and asthma also accompany warmer conditions.  In 2012 the U.S. suffered widespread drought and its hottest year on record.

In the past four decades there has been an increasing frequency of high-humidity heat waves, which are characterized by the persistence of high nighttime temperatures.  When the air stays extremely warm at night, there is less overnight relief, a fact that affects the young, elderly and ill particularly.  The percentage of land area in the United States with unusually hot summer nights has increased from an average under 10 percent in the 1970s to over 40 percent in recent years.
Warmer winters also allow insects and diseases to survive with dramatic consequences.  The successful overwintering of pine beetles, for example, in the warming winters of the Rocky Mountains contributed to the death of 46 million acres of trees from 2000 through 2012.

Paradoxically, perhaps, in winter, warming can also create increased snowfall.  Warmer winters reduce sea and lake ice, increasing so-called lake-effect snows in places like Buffalo.

Extremes are also the most dangerous aspect of rising sea levels.
The Jersey shore after Superstorm Sandy. (Credit: Anthony Quintano, CC BY-SA) Click to Enlarge.
For sea level, it is not the gradual increases that matter because we barely notice gradual increases in global mean sea level.  Rather, it is a storm surge on top of high tide on top of the rising sea level that causes devastation, as happened in the New York area and New Jersey shore in Superstorm Sandy.

The same is true of rainfall.

It is not the number of days with gentle showers that are of concern, but the increasing trend of torrential downpours – as witnessed just this week in Houston, where record-breaking April rains drove devastating floods.
Anticipating new extremes
It is important to note that our cities, our agricultural system, and our infrastructure are all built entirely around the weather conditions of the past.

In other words, changes in extreme weather, in any direction, can have a profound impact. Disaster often strikes when a threshold is crossed, and extreme events are precisely when this happens.  Adding climate change to natural variability in extreme weather can become the straw that breaks the camel’s back.

Read more at Has Climate Change Really Improved U.S. Weather?

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