What a Difference 0.5°C Makes! Or, How a Seemingly Small Amount of Global Warming can Lead to a lot More Rain

October 5, 2018
North Carolina National Guard
Astrid Caldas
Senior Climate Scientist

The soon-to-be released Intergovernmental Panel on Climate Change (IPCC) Special Report on Global Warming of 1.5°C (IPCC 1.5) assesses, among other things, the impacts that could be avoided if global warming is kept to 1.5°C instead of 2°C, and the ways we can limit some of the worst impacts of climate change and adapt to the ones that are unavoidable. The report is the result of an invitation by member countries of the United Nations Framework Convention on Climate Change (UNFCCC) during the adoption of the Paris Agreement, a worldwide commitment to reduce global warming emissions, which as we know has a goal of “Holding the increase in the global average temperature to well below 2°C above pre-industrial levels and pursuing efforts to limit the temperature increase to 1.5°C above pre-industrial levels, recognizing that this would significantly reduce the risks and impacts of climate change.”

Let us pause and think for a moment about this business of 1.5°C and 2°C, because 0.5°C just seems like such a small difference. Why so much discussion about this seemingly small difference in global temperature?

Let’s go back in time to look into the future, shall we? According to scientific and archaeological findings, the Medieval Warm Period (800 – 1400 AD) allowed Vikings to settle in Greenland around 1000 AD, but the Little Ice Age (between the 16th and 19th centuries) is believed to be behind the end of their prosperity – they couldn’t adapt enough to a changing climate. And these climatic changes were triggered by a temperature decrease of less than 1°C, taking place over a couple of centuries, which was enough to change things so much that the Vikings could not keep up with their farming ways.

Source: The lost Norse. Why did Greenland’s Vikings disappear? http://www.sciencemag.org/news/2016/11/why-did-greenland-s-vikings-disappear

Now, imagine what 1.5°C warming would do to our planet today. We better think about that because, as of right now, we have already seen a warming of 1°C globally from 1901 through 2016 – that’s over little more than one century, NOT over a couple of centuries like the Vikings experienced – and are on a path to approximately 3.5°C by the end of the century, which would be, by all measures, catastrophic to people, fauna, flora, and ecosystems.

That rapid rate of warming is already showing us that the impacts and consequences are numerous, for many people. Among those impacts are sea level rise, extended wildfire seasons, more frequent heat waves, and an increase in rainfall in many areas, including across the U.S. Let’s go a bit deeper on the topic of rainfall.

Physics fact: warmer air holds more water vapor. The potential consequence is clear: with more water vapor in the atmosphere, when it rains, there is a bigger chance of more rain – more frequent and/or more quantity. And the more the planet warms (up to 1.5°C, 2°C), the more water vapor can be stored, leading to potentially even more rain.

With global warming of 1°C to date, increases in atmospheric moisture content, changes in precipitation patterns, and intensification of heavy precipitation (the latter two over land) have been observed on a global scale. And these changes have been linked to carbon emissions from human activity (IPCC AR5), the main driver of global warming.

Another 0.5°C increase could make a sizable difference in the amount of rainfall we will see across the globe. Some areas will see an increase in annual precipitation while others are expected to see a decrease, and one thing scientists agree on is that wet areas will likely get wetter, and dry areas drier, especially in mid-latitude regions. The most recent IPCC report, the Fifth Assessment Report from 2014 (AR5), projected that, under a high-emissions, business-as-usual scenario, annual averaged precipitation in the equatorial Pacific, high latitudes, and many mid-latitude wet regions are likely to increase, while a decrease is projected for many mid-latitude and subtropical dry regions. Mid-latitude land masses and wet tropical regions are also very likely to see more intense and more frequent extreme precipitation events – even in areas where a decrease in annual precipitation is expected, there may be an increase in extreme precipitation events, meaning when it does rain, it is likely to be a heavier rain event. These predictions are especially for the Northern hemisphere, and we are certainly seeing them play out in the U.S.

Source: IPCC SR5 2014 https://www.ipcc.ch/pdf/assessment-report/ar5/syr/SYR_AR5_FINAL_full.pdf

According to the first volume of the Fourth National Climate Assessment, known as the Climate Science Special Report (CSSR), the U.S. has seen warming similar to the global average: 1°C from 1895 through 2016. Averaged annual precipitation in the U.S. has increased by 4% from 1901 to 2015, with great regional variation – some areas have seen increases higher than this average and some lower.  Region and seasonality affect the amount of rain change, both globally and in the U.S.

In addition to this overall increase in precipitation, rain events in the U.S. are also becoming more extreme as the air is almost completely saturated, a pattern similar to the rest of the world. The whole continental U.S. has seen an increase in the frequency of these heavy precipitation events between 1958 and 2016. Overall precipitation is projected to increase in some U.S. regions (such as the Northeast and Midwest) and decrease in others. In regions such as the Southwest, where an overall decrease is projected, when it does rain, it is projected to be a bigger rain event than previously.

Percent increase in the amount of rain falling during the top 1% of events per region in the continental United States between 1958 and 2016. The percentages are first calculated for individual stations, then averaged over 2° latitude by 2° longitude grid boxes, then averaged again over each region of the Fourth National Climate Assessment. Source: ucsusa.org/floods

According to the CSSR, for each 1°C increase in air temperature, the intensity of extreme precipitation events generally increases by 6%-7%.  Doing the math, with our current warming of 1°C, it is not surprising we have seen this increase in heavy precipitation events. If we get even warmer, more extreme rain events are likely. An increase of another 2°C would double the moisture content of saturated air to 12%-14%, and the intensity of extreme precipitation events could double. Now that’s an eye opener.

One important thing to keep in mind is that these changes are not coming by themselves. A change in precipitation comes with a change in temperature and other changes stemming from global warming. Together, the impacts can accumulate and/or act synergistically. For instance, a small decrease in precipitation paired with an increase in temperature is likely to lead to increased evapotranspiration, and less moisture retained in soils, with huge impacts for crops and natural vegetation. The consequences can be vast.

Extremes will keep getting more extreme as temperatures keep rising, and the IPCC 1.5 report should state it clearly. That is something we need to avoid, because we are already seeing enough (too many?) of these extreme precipitation events and the floods they bring, since heavy rainfall is one of the primary contributors to flooding. One need not go too far back to recall Ellicott City, MD, which saw two extreme precipitation events in two years, and Madison, WI, which was recently submerged by a rain event that exceeded the probability for a 6-hour, 24-hour, and 72-hour rain event. Both locations saw terrible floods. Keeping global warming to 1.5°C would likely reduce the probability of increases in heavy precipitation events compared to 2°C in the northern hemisphere, where they are more prevalent. This means that a difference of 0.5°C translates into a lower probability of heavy precipitation events in these regions.

Having warmed 1.0°C, we have 0.5°C to go before we reach 1.5°C averaged global warming, and 0.5°C more to reach 2°C. Each 0.5°C difference, especially brought on too fast, can mean countless (and some yet unpredicted) impacts around the globe. Even if global warming emissions were to drop to zero tomorrow, we are already committed to a series of impacts because of past emissions that will remain in the atmosphere for a century or more. The impacts of these past emissions are already playing out – some drastically – at this moment. Therefore, we must avoid continuing business as usual, and must instead focus on reducing emissions as much and as fast as we can, so as to reach net zero emissions by mid-century. This is the best, currently available way to avoid dangerous warming and otherwise irreversible impacts.