Much of the western half of the United States is suffering from severe drought of historic proportions. Conditions are particularly bad in California and the Southwest, but the drought extends into the Pacific Northwest, much of the Intermountain West, and even the Northern Plains.

Drought emergencies have been reported. Farmers and ranchers suffer. States are facing water shortages. Large forest fires burn earlier than usual. And there seems to be little relief in sight.

There are no precise parameters defining a drought, but it is commonly understood to be a period of unusually dry weather that lasts long enough to affect water supplies, agriculture, livestock, energy production, and other activities.

A drought usually starts with less than normal rainfall (and what is normal varies from region to region). If the drought persists, the courses of the rivers and the reservoir and water table begin to sink. Warm temperatures also have an influence, so that the snowpack melts faster in winter, which can affect the availability of water all year round. Excessive heat also causes increased evaporation from soil and vegetation, which can lead to crop failures and increase the risk of severe forest fires.

Experts from the United States Drought Monitor, a collaboration between several federal agencies and the University of Nebraska-Lincoln, rate the severity of the drought in a particular area and rate it from moderate to exceptional. They take into account many factors, including rainfall totals, snow cover, streams and soil moisture measurements, and use images from remote sensing satellites to assess the state of vegetation.

It’s very bad, both in terms of the size of the affected area and the severity. The latest map from the Drought Monitor shows that all or most of California, Oregon, Nevada, Arizona, New Mexico, Utah, and North Dakota have been drought and conditions are “severe” or “exceptional” in large areas of these states. Colorado, Idaho, Washington, Montana, South Dakota, and southwest Texas are also affected.

But cards only tell part of the story. The drought has had an enormous impact on the entire west, where the demand for water has risen sharply over decades as the population has grown.

In New Mexico, farmers along the Rio Grande have been told not to plant this year. Crop failures have been reported in Colorado and other agricultural areas. Lake Mead, the huge reservoir on the Colorado River, is so low that Arizona, Nevada, and other states are likely to face supply shortages. In North Dakota, ranchers transport water and food supplements for their cattle because the grazing land is so dry and the vegetation is stunted.

Conditions are particularly bad in California. Reservoirs in the state hold about half as much water as they do at this time of the year. The federal government has cut water allocations from its huge Central Valley project to California cities and farmers by 75 percent. And the Oregon border doesn’t have enough water for both endangered fish and farmers.

Wildfires of the size typically seen in summer have already occurred in California, Arizona, and New Mexico. Experts fear the forest fires will be severe and widespread this summer.

It is true that much of the west is usually hotter and drier than other parts of the country. Much of the Southwest and parts of Southern California are desert. Las Vegas, for example, rains an average of four inches per year, about one-tenth the national average. Much of the rest of California has a Mediterranean climate that can be wet in winter but hot and dry in summer.

Water has always been an issue in the West, and hundreds of billions of dollars have been spent on infrastructure in the last century to bring water from its source – mostly the mountains – to cities and farms.

It is also true that droughts in the region, like anywhere else in the world, are a normal part of life. They have occurred regularly over the centuries.

However, scientists say that climate change in the form of increases in temperature and shifts in rainfall is making the situation worse. What would be a moderate drought in a world without warming is more severe now.

Areas that would normally have had no rain for a few weeks can now go a few months without rain. In the mountains there is more precipitation than rain and not as snow, which decreases the snow cover. Even a decent blanket of snow now melts faster, making water supply difficult. And soils and vegetation lose more moisture with rising temperatures.

Meteorologists assume that it will last until summer. They forecast persistently hot and dry conditions in the west for the next few months.

Fall and winter are usually wetter in California and the Pacific Northwest, which can help. A pattern of summer storms known as the southwest monsoons can help in Arizona, New Mexico, and other areas. But the monsoons are unpredictable – the 2019 and 2020 monsoon seasons brought so little rain that they were labeled “nonsoons” and are one reason the drought is so severe this year.

But even if late 2021 and early 2022 are wetter in the west, the respite for much of the region can only be temporary. Another hot spring and summer next year could quickly deplete the snowpack, streams and reservoirs and dry out the soil again and the drought could return.

This has been the pattern in the southwest since 2000. Although there were some wet years during that time – 2019 is the latest – the drought has persisted. It would take several wet years in a row to completely banish the drought, and while natural climate variability cannot rule out such a long period of wetness, climate change makes it less likely.

The drought has persisted in the southwest for so long that some scientists say a mega-drought is developing in the region, similar or perhaps worse than the one that occurred in the past 1200 years and lasted for 40 years.