A megadrought is a severe drought that lasts not for months or a few years, but for decades. These are multidecadal periods of below-average precipitation and reduced streamflow that dwarf the droughts most people experience in a lifetime. The western United States has been living through one since the year 2000, and the period from 2000 to 2021 was the driest 22-year stretch the region has seen since 800 CE.
How Long a Drought Must Last to Qualify
There is no single, universally agreed-upon cutoff, but the term “mega” distinguishes these events from ordinary droughts by two key features: they persist for multiple decades, and they cover a much larger geographic area than shorter dry spells. The benchmark droughts of the 20th century, like the Dust Bowl of the 1930s and the severe drought of the 1950s, lasted roughly a decade or less. A megadrought surpasses both in duration and reach.
Scientists measure drought severity using indices like the Palmer Drought Severity Index, which tracks soil moisture relative to what’s normal for a given area. On that scale, a value below negative four signals extreme drought. Megadroughts don’t necessarily pin the needle at the worst possible reading every single year. Instead, they represent a prolonged stretch where dry conditions dominate and brief wet periods never fully replenish water supplies before the next dry spell arrives.
What Drives Decades of Dryness
Ordinary droughts can be triggered by a single season of weak rainfall. Megadroughts require something more persistent, and the tropical Pacific Ocean plays a central role. Research using tree-ring records spanning over a thousand years has found that megadroughts over western North America consistently coincide with 10-to-30-year stretches of unusually frequent La Niña-like conditions, where sea surface temperatures in the central and eastern Pacific run cooler than normal. During these cold phases, weather patterns shift in ways that steer moisture away from the western half of the continent. A striking 96% of megadrought years in the tree-ring record correspond to these cold Pacific conditions.
The Atlantic Ocean adds another layer. Warmer-than-average sea surface temperatures in the tropical North Atlantic can suppress summer rainfall over the American West by altering how air circulates over the region. Warm Atlantic conditions during the Medieval Climate Anomaly (roughly 1100 to 1300 CE) likely contributed to the especially brutal megadroughts of that era. The combination of a cool Pacific steering winter storms away and a warm Atlantic dampening summer rain creates a one-two punch that can lock a region into dryness for generations.
The Role of Climate Change
Natural ocean cycles can trigger megadroughts on their own, as they did repeatedly over the past 1,200 years. But the current southwestern megadrought has a new ingredient. An analysis published in the journal Science estimated that human-caused warming accounts for roughly 46% of the drought’s severity between 2000 and 2018. Higher temperatures pull more moisture from soil and vegetation, intensifying whatever dryness natural cycles deliver. Without that warming, the early 21st century would still have been dry, but only moderately so. Anthropogenic heating pushed it onto a trajectory comparable to the worst megadroughts of the past millennium.
Even under more conservative assumptions about how plants respond to higher carbon dioxide levels, at least 30% of the current drought’s severity traces back to warming caused by greenhouse gas emissions. In the climate models used for that analysis, 81% simulated some degree of human-caused drying in the region.
Megadroughts Are a Global Phenomenon
Western North America gets the most attention, but the paleoclimate record reveals megadroughts on every inhabited continent over the past 2,000 years. Central America experienced the so-called Terminal Classic Drought from roughly 800 to 1100 CE, a period that overlaps with the decline of major Maya cities. Chile and Argentina endured megadroughts around 1050 to 1200 CE and again from 1250 to 1400 CE. Europe, East Africa, and Eastern Australia all show evidence of multidecadal dry spells between roughly 1000 and 1300 CE. Northern China suffered repeated megadroughts from 1100 through 1600 CE, and Southeast Asia experienced one in the mid-1300s.
These events were not all connected by a single cause, but many clustered during the Medieval Climate Anomaly, when shifts in ocean circulation patterns affected rainfall regimes worldwide.
What a Megadrought Does to Water Supplies
The defining danger of a megadrought is that it outlasts the reserves meant to buffer against dry years. Reservoirs draw down, snowpack shrinks, and the underground aquifers that farmers and cities depend on as a backup begin to empty faster than they can refill.
California’s Central Valley offers a concrete example. Groundwater provides two-thirds or more of irrigation water during drought, and satellite measurements from NASA’s GRACE missions show that the rate of groundwater depletion in the Central Valley has been accelerating since 2003. From 1961 to 2021, the valley lost groundwater at an average rate of about 1.86 cubic kilometers per year. From 2003 to 2021, that rate climbed to 2.41 cubic kilometers per year. And from 2019 to 2021, during especially intense drought, it surged to 8.58 cubic kilometers per year. The pattern is striking: short periods of partial recharge followed by longer stretches of loss, producing a steady downward trend over two decades. Falling water tables dry up wells, cause the land surface to sink, and create a feedback loop where depleted aquifers actually worsen regional drying.
Economic and Agricultural Fallout
The financial toll compounds over time. In California alone, the 2021 drought directly cost the agricultural sector about $1.1 billion and nearly 8,750 jobs. When ripple effects on other industries are included, total losses reached an estimated $1.7 billion and over 14,600 jobs. Roughly 395,000 acres of farmland were left idle because there simply wasn’t enough water to irrigate them, with 385,000 of those acres concentrated in the Central Valley.
Those are the numbers from a single bad year within a longer megadrought. Stack enough of those years together and the losses reshape entire regional economies. Farmers shift to less water-intensive crops or stop planting altogether. Rural communities that depend on agricultural jobs shrink. Municipalities face harder choices about rationing residential water supplies. Infrastructure built for a wetter era, from canals to hydropower dams, operates below capacity or becomes unreliable.
Where Things Stand Now
The southwestern megadrought that began around 2000 has not fully ended. As of late August 2025, 65.5% of the western United States was classified as being in drought, with 14% in extreme or exceptional drought. The Colorado River Basin, which supplies water to roughly 40 million people across seven states and Mexico, was 100% in drought. Federal agencies have launched a special communication effort running through September 2026 to alert communities and economic sectors across the West about the risk to water supplies.
Wet winters can temporarily ease conditions, refilling reservoirs and boosting snowpack. But the defining characteristic of a megadrought is that these reprieves don’t last. The long-term trajectory remains dry, and the combination of natural ocean cycles and ongoing warming means the region’s water deficit continues to deepen even when individual years bring relief.