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2.2 Extremes

Heat leads to more extreme and unpredictable weather.

Narrated by
  • Devika Bakshi

The word volatile characterizes something that is likely to change in a very sudden or extreme way. In everyday language it can describe rapidly changing financial markets, or a person whose behavior can be unpredictable. In chemistry, it refers to a substance that vaporizes readily. By any of these definitions, water can be quite volatile. It can quickly vaporize, crystallize, condense, accumulate, rise, or travel. In the form of storms, floods, and cyclones, it can move with force and speed that can overwhelm anything in its path.

At low temperatures, H2O molecules gather to form ice or snow crystals. At high temperatures, they fly unattached as vapor. In between, they move in a consistently spaced liquid flow.

Early in human history, our ancestors could only explain extreme events like floods and storms as acts of god—there was no way to know when or how often they would occur, and what the magnitude of those events might be. Today, we understand much more about what causes these kinds of extreme events, and we have ways to forecast and track them. How might a warming world affect those patterns?

Redefining extremes

Water has a close collaborator: heat. Heat is a catalyst for water to shapeshift and move, melt and evaporate, and create humidity. Excess energy from a warmer atmosphere fuels every kind of water-related climate change impact, from storms to floods to global sea level rise.

In a stable climate, extreme events happen within known bounds. Climate stability allowed us to create measurements like a “1-in-100-year storm” and reliably build our regulations, infrastructure, and insurance markets around them.A warming atmosphere makes our previous measurements and expectations unreliable. Are we prepared for increasingly volatile weather? What will happen to our homes, food supply, and infrastructure when the definitions of average and extreme are continually changing?


Flooding has always been a part of the human story. Floods originate from natural phenomena like storm surges, deluges of precipitation, and swollen rivers. Our built environment—often intended to protect us from water—can increase flooding risks. Impervious surfaces like concrete prevent the ground from absorbing water, and levees and dams can overflow and break.

As the world becomes more populous and our cities become more crowded, flooding becomes an even bigger risk. Floods can displace large numbers of people, render land uninhabitable, and disrupt entire economies. Because it can happen almost anywhere, flooding has affected more people than any other environmental hazard. How we manage flood risks under a changing climate can mean the difference between famine and prosperity, life and death.


Compared to rainfall, storms are rare, and their violence makes them difficult to measure. As a result, we have less information about them, which makes them much harder to predict. A changing climate adds to this difficulty.

Tropical cyclones are caused by warm, moist air rising from the ocean’s surface and meeting cooler air in the atmosphere. The resulting swirling motion causes the storm to pull even more moisture and energy from the ocean’s surface.

Even so, there are things we can anticipate with confidence. We know that a warmer ocean and a more humid atmosphere create increasingly ideal conditions for powerful storms. Scientific evidence has shown that climate change is already causing tropical cyclones to intensify more rapidly, which leaves precious little time to prepare for an emergency.In communities where disaster preparedness resources are scarce, the effects of storms that are more powerful, and harder to forecast, can be especially catastrophic.

Breaking a system we rely on

Water is a global system. Rain, storm surges, rivers, oceans, and the atmosphere are all connected. The impacts of a warming climate go beyond extreme precipitation events, and we are now seeing more evidence of how it affects the balance of the ocean. As Greenland warms, its glaciers melt, adding freshwater to the salty ocean. We understand that more water in the oceans causes sea level rise, but the effect on ocean currents is more complex.

In the North Atlantic, Greenland’s melting ice sheet is already pouring massive amounts of cold freshwater into a key juncture in the Atlantic Meridional Overturning Circulation (AMOC)—the ocean pattern that helps regulate the temperate climate in Europe and the North American eastern seaboard, as well as some monsoon cycles in other parts of the world.

The AMOC plays a vital role in the global climate system. Like a big conveyor belt, it transports heat from the tropics to more northern latitudes such as North America and Europe. Without it, much of Europe would be significantly colder, and nearby farming areas much less suitable for food production.

Scientists have dubbed the cold ocean surface temperature anomaly near Greenland’s melting ice sheet the “cold blob.” This image from NASA shows 2015 mean global temperatures compared to a historical baseline of 1951-1980. While most of the world has gotten warmer, this spot has gotten cooler over the same period of time. Scientists think this cold freshwater might be starting to affect the strength of the AMOC.

As melting continues, the amount of freshwater in the North Atlantic may reach a still unknown tipping point. At that point, the system is at risk of collapsing altogether—potentially irreversibly—in a matter of decades.

The consequences of a collapsed AMOC would be widespread and destabilizing. The northern hemisphere would be colder, especially in places in the North Atlantic. Some monsoon cycles would dramatically shift. Europe would have erratic winter weather and less arable land. Sea level rise could accelerate in some areas, exacerbating flood risk. The impacts on our food supply, our economies, and our health are much harder to predict, but we can say with certainty that such a systemic change in the climate would be unprecedented for our civilization.

Scientists have already observed a slowdown in the AMOC. Without dramatic action to reduce emissions now, we may soon pass a threshold that would trigger a cascade of melting and subsequent climatic changes.

Building resilience for a volatile world

Our societies are finely tuned to the patterns and predictability of water in a stable climate. How will we cope with an atmosphere that unleashes water in increasingly erratic and volatile ways?

Fortunately, we understand the fundamentals of climate change and have tools to build more resilient communities. Climate models show us that we can expect more uncertainty in a changing climate. Some years will be unusually wet and some unusually dry; big storms that were once rare will happen more frequently, and we won’t know when exactly they will strike. We need to prepare our infrastructure, our finances, and our expectations for a wider range of outcomes. It’s within our power today to build resilience for the future, prepare for the events that are probable, and prevent those with irreversible impacts.