The undeniable science of extreme weather
Dr. Kevin Trenberth explains how global warming is supercharging droughts, floods, and storms
My friend and colleague Dr. Kevin Trenberth has written a guest post about climate extremes. Kevin is one of the most distinguished atmospheric scientists in the world and a well-known communicator of climate science. He’s best known for his work on energy flows through the climate system and how climate change is modifying it, but he’s done a huge amount of important work in many other areas of atmospheric sciences.
Given the misinformation spread by the DOE Climate Working Group report, Kevin’s perspective on extreme weather is extremely timely. AED
by Kevin Trenberth
A new analysis issued by the National Academies of Science, Engineering, and Medicine (NASEM) found that the evidence linking rising greenhouse gas emissions to negative human health outcomes is “beyond scientific dispute.” Climate change is real and it has already resulted in major damage.
The main cause is increasing atmospheric greenhouse gases of carbon dioxide, methane and nitrous oxide, all from human activities. Because carbon dioxide has a very long lifetime (hundreds to thousands of years), it is cumulative emissions that matter and the U.S. is the biggest contributor (although China, with a population 4x bigger than the U.S., has been a bigger annual contributor for the last two decades).
Carbon dioxide concentrations (of 425 ppm annualized in 2025) measured at Mauna Loa (Hawaii) have increased by over 50% relative to pre-industrial values (of 280 ppm).
These aspects have been well documented and understood for many years, and, with some hiccups, led to the Paris Agreement in 2015, a legally binding international treaty on climate change, adopted by 195 parties to limit global warming to well below 2°C, preferably to 1.5°C, compared to pre-industrial levels.
Not only have temperatures already risen by 1.5°C, mainly since the 1970s, increases in heat waves have also occurred and caused substantial damages. Many other extremes have also increased and are related to global warming, but in much less obvious ways. As temperatures rise, it seems fairly reasonable that there will be more high temperatures. But it is more than the overall rise in temperatures that is in play. Changes also relate to location, especially land versus ocean, and weather and weather patterns.
As temperatures rise, the water holding of the atmosphere increases by about 7% per °C (a physical law called the Clausius-Clapeyron equation), and this is observed to be happening over the oceans (where the supply of water is unlimited). The relative humidity tends to remain about the same on average. The reason is that rain events occur when the relative humidity exceeds about 85%. So if there is too much moisture, it rains out the excess. If it is too dry, then it doesn’t rain and evaporation occurring at the surface increases relative humidity.
Moreover, because the amount of moisture depends on temperatures, the values differ enormously with latitude and height. In mid-latitudes, typical column amounts are about 2.5 cm (an inch), but values can easily be double that in the tropics and subtropics, or less than half at high latitudes. The near-global average atmospheric water vapor has increased by 7% since the 1990s (Figure below) but these numbers are dominated by the tropics and vary with phenomena like El Niño.
Weather systems reach out and gather in moisture typically from over a distance about four times (3 to 5) the diameter of the precipitating area. The biggest rainfalls occur then when an atmospheric river or winds bring in moisture from lower latitudes and especially tropical regions. Tropical storms and hurricanes are accompanied by copious rainfalls. In this way, rainfall rates can greatly exceed that expected simply from emptying the atmosphere of moisture in a spot.
Moreover, as the moisture condenses to form rain, it gives back to the atmosphere the latent heat that was used to evaporate the moisture in the first place. This adds buoyancy and rising motions to the storms, and further enhances rainfalls.
Because land temperatures are rising faster than ocean temperatures (Figure above), as air moves from the ocean to the land as part of the hydrological cycle (ocean evaporation, wind moisture transport to land, rain on land, runoff and flow of water in rivers back to the ocean), the relative humidity of air drops. This is now clearly observed on land even as the total moisture has increased. Because onset of rainfall depends on relative humidity, rain events on land become less frequent. But when rain events are triggered by weather systems, they become larger, more intense and more likely to cause flooding, as is also observed to be happening.
The more technical details matter and are briefly outlined here. Heat from the Sun occurs mainly at the surface, and nearly all weather systems (clouds, thunderstorms, hurricanes, cyclones) systematically move heat upwards to where it can be carried around by winds and ultimately radiated back to space.
Such events cannot happen if the atmosphere is stable, as in strong anticyclones. Thus, the vertical temperature structure of the atmosphere matters. Meteorologists have a metric for how stable the atmosphere is: Convective Inhibition (CIN). Meanwhile the tendency for near-surface warm moist air to destabilize and start to rain is measured by another metric: Convective Available Potential Energy (CAPE). Both CIN and CAPE vary spatially, especially with monsoons, the Hadley Circulation and the Walker Circulation, throughout the tropics and subtropics.
With climate change, CIN tends to increase over continents because of lower relative humidity, while CAPE tends to increase nearly everywhere, especially because of increased surface evaporation and low level moisture amounts. Where it is not raining, the result is longer dry spells, increased droughts, heatwaves and wildfires. Where it is raining, there is a greater risk of heavy rains, or snow, and greater risk of floods. Water is the great air conditioner! All of these phenomena increase with global heating and cause huge disruption and costs, as well as loss of life.
Scientific facts, understanding, and observations provide ever increasing support that climate change is real, as detailed in the NASEM report. But deniers of climate change are ignorant of observed facts and physical explanations of what is actually going on. The normal variability of weather and climate variability such as El Niño, can make the records messy but the reasons are well understood.
In the absence of progress toward sufficient decarbonization to rein in human activities that cause climate change, which is very much a global problem, the only option for small countries and individuals is to plan for the consequences. This is so-called adaptation and building resilience. It requires paying special attention to rising sea levels and coastal inundation, drainage systems to deal with torrential rains and flooding, managing water to deal with droughts and wildfire risk, and protection from heat waves. Increasing extremes in weather are already occurring and prospects are for more of the same.
other stuff you should read
Climate Cafe: Five forecasts early climate models got right — the evidence is all around you
This post is long but one of the best I’ve ever read at explaining how the electrification revolution will change the world: The Electric Slide on Not Boring by Packy McCormick
Just a slight comment on a statement in the second paragraph. It is the atmospheric perturbation of the CO2 concentration that has a long lifetime. The lifetime of a particular CO2 molecule in the atmosphere is only 4-5 years or so, being taken up by the active terrestrial biosphere or upper wind-mixed layer of the ocean. There are many examples of confusion about this and I personally think we ought to be a bit more precise in making this clear.
Should we not emphasize the need for adaptation to the effects of CO2 accumulation that has already occurred. not just to the accumulation in the future that will be affected by policy?