PAST: warming, droughts, and wildfires in Maricopa County

11/15/2017

Average annual temperatures in Phoenix, the county seat of Maricopa County, 1935-2017. Source: NOAA Climate at a Glance.

Climate changes and the local environment:

Since 1970, average annual temperatures in Arizona have soared. In densely-populated Maricopa County, which includes Phoenix, average annual temperatures have risen by over 7° F (nearly 4° C), which is roughly four times the global average. In no major U.S. city have temperatures climbed more quickly.

This remarkable rise has sharply increased the frequency of particularly extreme weather. In the summer of 2017, for example, a heat wave sent temperatures soaring to over 120° F (nearly 50° C). Summer days with relatively cool temperatures, by contrast, have grown scarce. From June to September 2017, only 14 days had high temperatures lower than 100° F.

Higher temperatures have increased rates of evaporation across Arizona, and therefore dried much of the state. Average annual precipitation in Maricopa County has actually increased slightly over the past century, yet precipitation in mid to late summer has declined substantially. Summer droughts have therefore become common in Maricopa County.

Warming, drying trends have created ideal conditions for wildfires across Arizona, including in Maricopa County. Wildfires in Arizona have therefore grown more frequent, and they last longer. In spring 2017, a wildfire in Maricopa County’s Tonto National Forest consumed some 500 acres of woodland.

Drought severity in Maricopa from 2000 until 2017. Darker colors represent worse droughts; higher values mean more of the county was in drought. Source: United States Drought Monitor.

How we know:

These reconstructions are largely based on reliable, easy to use tools that are freely accessible online: the National Oceanic and Atmospheric Administration (NOAA) State Annual and Seasonal Time Series, the NOAA Climate at a Glance app, the National Drought Mitigation Center's United States Drought Monitor.

NOAA's State Annual and Seasonal Time Series and Climate at a Glance tools use data collected at weather stations and stored in NOAA's U.S. Climate Divisions database. The Drought Monitor compiles weekly maps of drought conditions across the United States, and combines climate, soil, and water data with reports from 350 experts across the country.

Average daily maximum air temperature in Phoenix, 1934-2017. Source: NOAA Climate at a Glance.

Human consequences:

Rising summer temperatures in Arizona have increased the number of heat-related deaths in Maricopa County. Especially hot, dry weather can provoke dehydration and heat stroke, and can interfere with cardiovascular, respiratory, and nervous systems. The very old, the very young, and the sick are especially vulnerable, particularly in poor neighborhoods where some households might not have dependable access to air conditioning. Employees who spend most of their time outside - such as construction workers - are also at risk.

Heat-related threats to human health can be especially great in cities like Phoenix, where paved surfaces create urban heat sinks that amplify hot temperatures. In 2016, Maricopa County endured 130 heat-related deaths, the most in the last 15 years. On average, Arizona hospitals now treat almost 2,000 patients per year for heat-related illnesses. Many injuries and deaths not only reflect warming trends, but also social and infrastructural problems that make it hard for people to escape the heat.

Wildfires in Arizona, including in Maricopa County, have also proven costly for state and federal governments. Containing the Arizona wildfires has cost many millions of dollars, and the total costs of wildfires can exceed suppression costs by more than 30 times. In Maricopa County, smoke billowing from fires in Tonto National Forest, for example, shut down Bush Highway. Foresters have conducted controlled burns to limit the future risk of major wildfires in the forest, but prolonged drought has foiled some of these efforts.

In Phoenix, the municipal government has taken steps to lower the city's contribution to climate change. It has expanded public transit, constructed new bike lanes, purchased electric vehicles, installed low-energy bulbs in street lighting, and set ambitious goals for cutting greenhouse gas emissions. Still, adapting to climate change has proven to be an even more daunting task, especially for the city's less affluent residents.