Has climate change really improved US weather?

By Kevin Trenberth
National Center for Atmospheric Research

According to a new report published in “Nature” on April 20 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.

Egan and Mullin do note that this trend is projected to reverse over the course of the coming century, and that Americans will become more concerned about climate change as they perceive more negative impact from weather. However, they estimate that such a shift may not occur in time to spur policy responses that could avert catastrophic impacts.

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.

At the National Center for Atmospheric Research, my colleagues and I have conducted numerous studies analyzing how climate change is altering regional, national and global weather patterns.

Many of those studies focus on extreme events such as floods, hurricanes, heat waves and droughts because these are the weather phenomena that have major impacts and costs: they destroy crops, wreck infrastructure and threaten lives and property.

Analyzing the impact of climate change by focusing on average weather patterns greatly underplays climate change impacts and may make Americans dangerously complacent about how climate change is already affecting our lives.

Impacts of climate extremes

Egan and Mullin claim that “80 percent of Americans live in counties that are experiencing more pleasant weather than they did four decades ago.” They attribute this change to rising winter temperatures paired with summers that have not become “markedly more uncomfortable.” The result, they conclude, is that weather has shifted toward a temperate year-round climate that Americans have been demonstrated to prefer.

For their investigation of temperature trends, the authors looked only at the average of temperatures reported in the months of January and July. For precipitation trends, the authors looked only at annual precipitation totals and the number of days on which precipitation occurs annually.

But people don’t live in annual or monthly averages!

The effects of climate change are mainly manifested through changes in extremes, because the biggest impacts, loss of life and damage to property occur especially in those conditions that break records and go beyond previous experience.

But the Egan and Mullin paper does not account adequately for extremes. Moreover, it should also be noted that people care about weather year-round, not just in January or July.

Warming is evident in all seasons but the most outstanding high values are for March-April-May 2012 and September-October-November 2015, not just January and July. Adapted from NOAA data. | Author provided

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 2015 wildfire season was the largest in Washington state history. | MODIS

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.

Yes, it is likely true that some Americans prefer warmer winter conditions. Skiers and others who love winter sports, however, are not in that group, and more significantly, in many places, including California, warmer and drier winters have helped to drive long-term drought. Last winter, for the first time in 120 years of record-keeping, the winter average minimum temperature in the Sierra Nevada mountains was above freezing. Across the state, the prior 12 months were the warmest on record.

As a result, the Sierra Nevada snow pack that normally provides nearly 30 percent of California’s water stood at its lowest level in at least 500 years, despite modest increases in precipitation from the record lows of preceding years. The few winter storms of that year were warmer than average and tended to produce rain, not snow. What snow fell melted away almost immediately.

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.

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 Jersey shore after Superstorm Sandy. | Anthony Quintano, CC BY-SA

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.

The fact is that over the past century the U.S. has, on average, witnessed a 20 percent increase in the amount of precipitation falling in the heaviest downpours, with a 71 percent increase in the Northeast region and a 37 percent increase in the Midwest. This surge of extreme precipitation has dramatically increased the risk of flooding, especially in the regions with the largest increases in heavy precipitation.

In a warming world, storms become stronger and rainfall more intense owing to more moisture residing in the warmer atmosphere. Torrential rains flooded much of South Carolina,
for example, last October, and Missouri experienced unprecedented rains in November and December 2015, resulting in flooding along the Mississippi River.

In May 2015 it was Texas and Oklahoma that experienced record rains and flooding, perhaps influenced by the major El Niño event combined with global warming. In September 2013 it was Boulder and the Front Range of the Rockies that suffered from major flooding arising from heavy prolonged rains.

These examples show that climate change makes itself felt throughout the year.

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.

As detailed above, extreme weather has an outsized impact on everyday life. Ignoring the impact of extreme weather in determining the trend in “pleasant” weather conditions is, I would argue, nonsensical. Indeed, the trends in heat waves, drought and extreme precipitation would all seem to indicate that the weather overall has become more unpleasant and difficult to deal with.

Kevin Trenberth, Distinguished Senior Scientist, National Center for Atmospheric Research.

This article was originally published on The Conversation.The Conversation

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