Droughts are caused by prolonged periods of below-normal precipitation, but the forces behind that rainfall deficit vary widely. Persistent high-pressure weather systems, ocean temperature shifts, climate change, and even human land use all play a role in triggering or worsening drought conditions. Most droughts result from several of these factors combining at once.
How High-Pressure Systems Block Rain
The most immediate cause of any drought is a stubborn area of high atmospheric pressure that parks over a region and refuses to move. Under high pressure, air sinks toward the ground. Sinking air warms as it descends, which prevents moisture from rising, cooling, and forming clouds. The result is clear skies, day after day, with storm systems forced to detour around the high-pressure dome.
Meteorologists call these stalled patterns “blocking events.” A blocking high can sit over an area for weeks or even months, shifting the jet stream and the storm track away from the affected region. While the area under the block bakes in hot, dry weather, neighboring regions may actually experience unusual flooding as diverted storms pile up elsewhere. When a blocking high lasts long enough, the dry spell crosses the line into drought.
Ocean Temperatures and Rainfall Patterns
The tropical Pacific Ocean acts as a global thermostat for rainfall. During El Niño events, warm water builds up along the equator in the eastern Pacific. That warm surface heats the air above it, fueling heavy rainstorms over the eastern Pacific and parts of South America while pulling moisture away from places like Australia, Southeast Asia, and parts of Africa. During the strong 1997 El Niño, as much as 12 millimeters more rain than average fell over the eastern Pacific, while the southeastern United States got wetter than normal.
La Niña flips the script. Cool water pools in the eastern Pacific, and the atmosphere responds by producing less evaporation and fewer storms. Ecuador, Peru, and the southeastern United States tend to dry out during La Niña years. These swings between El Niño and La Niña, known collectively as ENSO, are one of the most powerful natural drivers of drought worldwide. A prolonged La Niña can push vulnerable regions into severe, multi-year drought.
The Self-Reinforcing Drought Cycle
Once a drought begins, the land itself can make it worse. Dry soil absorbs more solar energy than moist soil because less water is available to evaporate and carry heat away from the surface. That extra heat goes directly into warming the air near the ground. Warmer, drier air near the surface makes it harder for clouds to form, and the clouds that do develop tend to sit higher in the atmosphere and let more sunlight through to the ground below. This creates a feedback loop: dry soil leads to hotter temperatures, fewer clouds, less rain, and even drier soil.
Research using models that simulate the full water cycle from groundwater to cloud tops shows that drought conditions physically alter cloud properties. In dry initial conditions, the land surface absorbs significantly more energy, and that energy gets converted almost entirely into heat rather than evaporation. In regions that are already dry, this feedback kicks in quickly because there is so little surface moisture to buffer the cycle. It’s one reason droughts can intensify rapidly once they take hold.
Climate Change Is Making Droughts Worse
Rising global temperatures don’t just raise the thermostat. They fundamentally change how water moves through the atmosphere and landscape. The Intergovernmental Panel on Climate Change concluded with high confidence that extreme droughts have become more likely and more severe due to human-caused warming. The total land area experiencing more frequent and intense drought is expanding, and in the Mediterranean, southwestern South America, and western North America, the drying trend already exceeds anything seen in the last thousand years.
The projections are stark. At 1.5°C of global warming above pre-industrial levels, the likelihood of extreme agricultural drought is projected to at least double across large parts of northern South America, the Mediterranean, western China, and high latitudes in North America and Eurasia. At 2°C of warming, that likelihood jumps by 150 to 200 percent. At 4°C, it rises above 200 percent. Higher temperatures increase evaporation from soil and plants, meaning that even when rain does fall, more of it is lost to the atmosphere before crops or reservoirs can use it.
Deforestation and Land Use
Forests are rainfall engines. Trees pull water from the soil and release it into the atmosphere through their leaves, a process that generates moisture for clouds and downwind precipitation. When large tracts of forest are cleared, that moisture recycling breaks down. A study published in Nature found that tropical deforestation causes measurable drops in local rainfall, with the effect growing stronger at larger scales. At scales of 200 kilometers, each percentage point of forest loss reduced precipitation by about 0.25 millimeters per month.
The consequences are particularly severe in the tropics. In the Congo Basin, researchers estimate that continued deforestation will reduce local precipitation by 8 to 10 percent by the year 2100. The Amazon showed declines of about 0.23 millimeters per month for each percentage point of forest cleared, while Southeast Asia saw even steeper drops of nearly 0.48 millimeters per month. These numbers may sound small on a monthly basis, but compounded over years and across vast regions, they translate into meaningfully drier conditions that push landscapes toward drought.
Groundwater Overuse Deepens the Problem
When natural rainfall fails, communities and farms often turn to groundwater to fill the gap. But pumping aquifers faster than they recharge can transform a temporary dry spell into a long-term water crisis. Central Chile offers a cautionary example. The region has been in its most severe drought since 2010, and the government responded by ramping up groundwater extraction by 383 percent between 1997 and 2022. That pumping, not the drought itself, became the dominant force behind falling water tables. Researchers found that groundwater withdrawals accounted for roughly 65 percent of the total drop in aquifer levels during the drought period, while the actual rainfall deficit was responsible for only 35 percent.
This kind of over-extraction creates a new, more complex form of drought. Even if rainfall returns to normal, depleted aquifers can take decades or longer to recover. Rivers and wetlands that depend on groundwater dry up, ecosystems collapse, and communities that relied on wells find themselves without a backup water source.
Different Types of Drought
Not all droughts look the same, and scientists distinguish between several types based on what part of the water cycle is affected. Meteorological drought is the most straightforward: it simply means a region is getting significantly less precipitation than normal for a sustained period. The definition varies by region because “normal” rainfall in the Amazon looks nothing like “normal” rainfall in Arizona.
Agricultural drought develops when soil moisture drops low enough to stress crops, which can happen even before a meteorological drought becomes severe. If topsoil moisture is still adequate during early growth stages, plants may survive a dry stretch, but if subsoil moisture is also depleted, yields will suffer. Hydrological drought refers to shortfalls in surface and underground water supplies: rivers, reservoirs, lakes, and aquifers. This type typically lags behind the others because it takes time for reduced rainfall to work its way through the watershed. A region can be in meteorological recovery, with rain finally returning, while still experiencing hydrological drought because reservoirs and aquifers haven’t yet refilled.
Understanding which type of drought is occurring matters because the impacts and responses differ. A farmer worries about soil moisture months before a city water manager sees reservoir levels drop. Each type unfolds on its own timeline, and a single prolonged dry period can trigger all of them in sequence.