ClimateBrief: Are There More Precipitation Extremes in a Warming Climate?

Introduction

This summer has been one of extremes: heat, drought, flooding, fire, and other weather-related disasters. Above, the title graphic shows the precipitation from one of the most recent examples of flooding this summer in the central part of TN. A record 24-hr precipitation record for the state (about 17”) was set at McEwan, in the left-center of the graphic. Media out of the area has been showing terrifying destruction from the huge amounts of rain in that area.

Meanwhile, about 50% of the U.S. is in slight, moderate, severe, extreme or exceptional drought.

The seasonal outlook for drought indicates that most of the north-central and western U.S. will continue to see the same or worsening drought conditions through at least 30 November; bad news for farmers and ranchers in places like WA, OR, CA, ND, MT, and northern MN.

While most of us are convinced that these extreme events are the result of human-caused global warming, it has been difficult until the last decade or so to attribute a single, specific event to it. Of course, the question asked, particularly by global warming deniers, is about what climate change scientists call attribution: “Are we really causing these floods (or droughts), or is this just Mother Nature being a real ‘mother’ (i.e. Isn’t it just natural variability)?” The rest of this story will discuss how we can (and cannot) attribute heavy precipitation and drought to global warming.

Heavy rainfall

For each degree Celsius the mean temperature increases, the atmosphere can hold 7% more water vapor. The maximum amount of water vapor that air can hold (saturation vapor pressure P_s) as a function of temperature is shown in the diagram below. P_s can be considered the maximum possible moisture available for precipitation at a given temperature.

This would seem to mean that global warming means more flooding rains (and heavy snows where it is cold enough), with more precipitation on average.

But, unfortunately, it’s not that simple. Having more available moisture for precipitation is a necessary but not sufficient condition for heavy rain or snow. There must be a mechanism to condense the water vapor into precipitation that will eventually to fall to the earth’s surface as rain or snow. That mechanism is to cool the air to its saturation point and to have dust or other particles on which the water vapor can condense, which is almost always accomplished through upward vertical motion. Forcing for upward motion can occur through winds blowing over a mountain range, through larger-scale atmospheric processes that force the air to rise, or through heating of air near the earth’s surface.

Upward motion and precipitation occurs on a variety of scales. This could be anywhere from a thunderstorm updraft less than 1 kilometer (km) wide, to large upgliding conveyor belts of moist air several 100s of kms across, associated with large mid-latitude storm systems.

The problem is thus that we must know the climatology of where, when, how, and how much the upward motion takes place at all these scales. Does this climatology also change as the result of global warming? To get a feel for this, climate scientists turn to atmospheric models run over a number of decades (typically out to the year 2100) to project possible impacts of increased greenhouse gas emissions.

Changes in the climatology of upward motion/precipitation in climate models

Mid-Latitudes and Polar Regions

Winter

The jet stream, which guides low-pressure systems that impact day-to-day weather, is driven by the temperature gradient between the poles and mid-latitudes/subtropics. It turns out that polar amplification of global warming actually increases the speed of the projected winter jet stream. This is because while the lower Arctic atmosphere (troposphere) is warming, the upper Arctic atmosphere (lower stratosphere) actually coolsenough to increase the jet-stream level pole to mid-latitude temperature gradient. Increased moisture from the rise in temperature could provide more energy and projected strengthening of mid-latitude storm systems.

The location of the winter jet stream also is projected to shift poleward; this also results in a northward shift of mid-latitude storm systems, implying increases in projected precipitation in areas north of the present storm track. An 8% increase in precipitation has already been observed in the Arctic over the past 100 years. In places dependent on winter precipitation, like the U.S. southwest, the Mediterranean region, and the Middle East, an overall drying is likely to take place because of fewer mid-latitude storms. This corresponds to recently observed drought and wildfire areas.

Summer

Mid-latitude storms are weaker and weather patterns move more slowly in summer than in winter. Summer stagnation of weather patterns persists for longer periods in climate projections, which can result in larger precipitation amounts (and flooding, as was seen in the Low Countries, France, and Germany in July), or longer dry periods which could lead to drought. On average, energy for thunderstorms increases in the lower atmosphere because of the additional moisture available from global warming, but this doesn’t translate to heavier total rainfall everywhere. At large distances from oceans or other large expanses of open water, the generation of warm season precipitation can become dependent on local vegetation and soil moisture conditions. Areas well inland that experience less winter storminess and a longer warm season, for example, may lose summer precipitation because

• There is less recharge of soil moisture in the winter season
• Vegetation is active for a longer period and depletes soil moisture more rapidly

This would lead to less moisture available for precipitation, especially later in the warm season.

Areas that are projected to be sensitive to local soil moisture sources include the U.S. Great Plains, central agricultural belt of Russia, and eastern Europe. Drought is moderate to extreme from MT to MN as I write this (see graphic above). These areas are all important food sources for the world.

What About the Tropics?

What will happen in the tropics is less clear, and may be as much driven by regional changes as in the large scale monsoon circulations in places like Asia, Africa, and Australia. As in the mid-latitudes and polar regions, storm systems such as tropical waves and tropical cyclones could have more energy because of the increased water vapor.

In the tropics, most computer climate projections suggest there may be a narrower and stronger “intertropical convergence zone (ITCZ)”. The ITCZ is a belt of upward motion, thunderstorms, and heavy precipitation resulting from the convergence of the near-surface northern hemisphere northeasterly and southern hemisphere southeasterly trade winds. Tropical disturbances of various scales may also be affected because of increased moisture availability to fuel them.

Some tropical regions are dependent on live vegetation as a moisture source. Climate models with removal of the rain forest in the Amazon project a permanent drying of the region, and changes in atmospheric circulation and precipitation patterns in areas far removed from the South America. In sum, the projected tropical future is a mixed bag with competing drying and moistening impacts from global warming.

So, What Does This All Mean?

If it were just a matter of the quantity of atmospheric water vapor, it would be simple to state that  increased rainfall is caused by more water vapor induced by global warming. However, global warming also changes the global circulation, which may act to intensify precipitation in some areas and reduce it in others. Condensation of water vapor releases heat into the atmosphere which can also feed back onto and further change the atmospheric circulation.

There are some changes suggested by climate model projections that have a high likelihood:

1. Poleward shifts in the mid-latitude jet stream
2. Poleward shifts in the mid-latitude storm track
3. Increased precipitation in the polar regions
4. Drying in areas with wet winters and dry summers, resulting from (1) and (2) above
5. Drying in many inland agricultural areas where moisture from local sources (e.g. vegetation moving soil moisture to the local atmosphere) is important
6. Increased mid-latitude precipitation from increased available water vapor and slower-moving warm season storm systems

Note that I’ve simplified much of this for public consumption; let me know if you have any questions in the comment section below and I’ll get to them over the weekend.

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