However, the increase in water vapor not only brings with it an increased risk of storms. It also makes summer nights uncomfortably muggy. Since the mid-1990s, nighttime low temperatures in summer have risen more sharply than daily high temperatures over land areas worldwide. This is also due to the greenhouse gas water vapor: Heat that would normally escape into space at night is trapped in the atmosphere, preventing the Earth’s surface from cooling. And unlike carbon dioxide, which spreads across the globe no matter where it is released, water vapor initially stays where it is created.
The increased humidity makes hot nights a health risk. Because the air contains more water vapour, sweat evaporates less, which makes it difficult for the body to cool down naturally. The result is overheating and sleep disturbances. A measure of these symptoms is the heat index: it describes the combined influence of temperature and humidity and quantifies the stress to which the body is actually exposed. For example, 33 degrees Celsius with 60 percent humidity results in a heat index of 40 degrees Celsius. A heat index of more than 38 degrees Celsius is considered dangerous. Prolonged exposure can be fatal, especially for the elderly and children. Livestock and domestic animals also suffer from nighttime heat, while wildlife flees to higher latitudes or higher elevations. In addition, when it cools less at night, heat can accumulate in the soil, killing some plants and insects while others—more warmth-loving species—perhaps thrive better. According to the “Declaration on Climate Change and Health” published by 32 US health organizations in August 2021, nighttime heat also increases the risk of insect-borne diseases.
Feedback amplifies the temperature rise
Exposure to high nighttime temperatures is increasing not only in the hot tropical regions, but also in areas well north or south of the equator. In Houston, USA, the average temperature today is more than two degrees Celsius higher than in 1970. This is due to the proximity of the Gulf of Mexico, which has become warmer, and the rapid growth of the city, which increases the urban heat island effect.
This coincides with the developments in Germany: the number of “tropical nights” in which the temperature does not fall below 20 degrees Celsius has increased steadily since the 1990s. In Hesse, for example, between 1961 and 1990 such a warm night was recorded about every five years. In Frankfurt am Main, the German Weather Service counted eight tropical nights in 2015, six in 2018 and four in 2019. In the future, these conditions will probably become normal.
However, some tropical countries will suffer the most – or are already doing so. In May 2015, India and Pakistan were hit by a severe heat wave. The maximum daily temperatures there were over 46 degrees Celsius for days, and due to the high humidity it could hardly cool down at night. According to the United Nations, around 3,500 people died as a result of this exposure. If global warming increases by another half a degree, the number of people at risk from extreme heat worldwide will increase to 500 million, estimated climate scientists from Rutgers University in the US in 2020.
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The three vibrational modes of the hydrogen molecule |
Every molecule in the atmosphere that absorbs infrared radiation acts as a greenhouse gas: it stores the energy radiated from the earth in the form of molecular vibrations. As a result, it does not escape into space, but remains in the atmosphere, causing it to heat up.
During molecular vibration, either bond angles or bond lengths change periodically – as if the atoms were spheres that are connected to each other by springs. The center of gravity of the molecule always remains the same, so it does not move in one direction due to the vibration. Each type of vibration (one speaks of vibration modes) requires a specific amount of energy to be excited, i.e. radiation of a corresponding wavelength.
The water molecule has three vibrational modes (right; white: hydrogen, red: oxygen): if the bond angle of the molecule changes, this is referred to as the H–O–H deformation vibration (above). In the symmetric O–H stretch (middle), the bonds between the oxygen and the two hydrogen atoms lengthen and shorten synchronously. In contrast, the lengths of these bonds always change in opposite directions in the antisymmetric O–H stretch (bottom).
While carbon dioxide receives the most attention in the context of global warming, water vapor is by far the most important greenhouse gas in the atmosphere. It absorbs much more of the infrared radiation emitted by the Earth’s surface than other greenhouse gases, thereby trapping more heat. But that’s not all, since warmer air can hold more water—so the water vapor content increases as the temperature rises—it amplifies any warming caused by carbon dioxide or other greenhouse gases. To illustrate the order of magnitude: a doubling of the atmospheric concentration of carbon dioxide would, taken by itself, heat the earth by about one degree Celsius. Due to feedback, however, the temperature rise is twice as high. However, other feedbacks, such as dwindling sea ice, get most of the attention. The water vapor feedback loop – warming leads to evaporation, trapping heat that causes further warming – is the strongest in the climate system.