In the middle of summer, most Southern Californians would be surprised to see more than a handful of clouds in the sky (unless you count the plumes of smoke from wildfires). But on July 14, a rapidly developing thunderstorm in Twentynine Palms dumped about 1.88 inches of rain in a matter of hours—a staggering amount by historical standards for a city that typically receives less than 4 inches of rain in an entire year.

Water rushed down streets and highways, sweeping away cars and carrying debris that damaged homes and businesses. While residents were still surveying the destruction, a flash flood warning was issued for the area nine days later as another thunderstorm passed through.

So why does the Mojave Desert—obviously an extremely dry place—get all the summer rain, while Los Angeles, less than 150 miles away, gets no rain at all?

Late summer is the driest season in much of California. At Los Angeles International Airport, August sees no rain at all on average. But head east over the mountains into San Bernardino County and the pattern suddenly changes: In Twentynine Palms, August is the wettest month. Continuing into Arizona and New Mexico, precipitation becomes even more intense in late summer: In Santa Fe, nearly half of the annual rain falls between July and September.

This change is the result of a phenomenon called the North American Monsoon, which was responsible for the flooding in Twentynine Palms. Although less well-known than its South Asian counterpart, the North American Monsoon plays an important role in the climate of the Four Corners states, bringing vital moisture to areas that would otherwise be bone-dry, but sometimes also causing destructive flooding.

Read more:Monsoon floods devastate city in the Mojave Desert – but also reveal its heart

All monsoons are powered by the same source: a temperature difference between land and sea. During the hot late summer months in the subtropics, the sun can deliver up to 1,000 watts per square meter – the equivalent of the power you would get from a space heater every four feet. Some of this power is reflected back into space, but on average more than two-thirds of it is absorbed by either the land surface or the sea.

The key difference between the two is that the ocean is constantly being mixed, which distributes heat throughout the top 18 meters of ocean water – something that is impossible on land. As a result, the top few centimeters of soil or rock heat up rapidly over the course of a day, in turn warming the air above it.

Because warm air is less dense than cold air, the air over land rises, typically in the late afternoon after a full day in the sun. As the air rises from the land, it draws in moist air from the water – in the North American monsoon, this is the Gulf of California – and replaces it. When this moist air reaches mountainous terrain such as that of northern Mexico and the American Southwest, it is forced upward and releases its moisture, often in sudden, violent thunderstorms.

There are many places near the coast that don’t experience monsoons – Los Angeles, for example. One important factor is topography: research suggests that a major reason for the strength and persistence of the South Asian monsoon is the presence of the Himalayas, which act like a wall, preventing air from the dry Tibetan plateau from reaching the Indian subcontinent.

Another important aspect is the so-called “subtropical ridge,” a series of persistent high-pressure areas that all occur about 30 degrees north (and south) of the equator.

The reason the North American monsoon does not reach the California coast is the presence of the North Pacific High, which is part of the subtropical ridge that typically lies northeast of Hawaii. The North Pacific High strengthens and expands during the summer, creating the hot, dry conditions typical of Los Angeles and crowding out the monsoon. During the winter, however, the North Pacific High tends to weaken and shift southward, allowing atmospheric rivers to reach the state.

For the southwest, the North American monsoon can be both a blessing and a curse.

It brings much-needed rain to the region, but it usually falls in torrential downpours that the dry, hard ground cannot absorb. This leads to dangerous flash floods that can destroy roads and buildings and potentially claim the lives of those who get in their way. The rain and cool conditions that the monsoon brings can be helpful in putting out wildfires, but the lightning from the storms is also a major trigger of wildfires in the region.

Read more:Heat waves are getting hotter and longer. What are the causes of extreme temperatures?

As with many weather phenomena, climate change is expected to have some impact on monsoon rainfall, but the magnitude and direction of this effect will depend on specific local factors.

In some parts of the world, such as South and East Asia, monsoons are expected to become more intense due to climate change. Changes in aerosol pollution due to China and India’s (hopefully) move away from coal power are expected to play a very important role.

In the Southern Hemisphere, models suggest a possible small increase in summer monsoon rainfall. Of the major monsoon systems, only the North American monsoon is expected to experience a significant decrease in total rainfall, with the most likely outcome being a 1 to 6 percent reduction in summer rainfall. The reasons for this projected decline are not fully understood, but rising sea surface temperatures off the coast of Baja California have been suggested as a possible explanation.

If the North American monsoon does weaken in the coming decades, it will place even greater stress on the shrinking Colorado River, whose basin includes most of Arizona and much of Colorado and Utah. Perhaps more seriously, it will pose a serious threat to ecosystems already fragile due to rising temperatures and the spread of the mountain pine beetle.

The future of the world’s monsoon systems is neither certain nor easy to predict. However, given the potential threats in both directions – more severe floods or worsening drought – it is important that we prepare for both scenarios and act quickly to limit these changes, including by rapidly reducing emissions.

Ned Kleiner is a scientist and disaster modeler at Verisk. He holds a PhD in atmospheric sciences from Harvard.

This story originally appeared in the Los Angeles Times.