With the onset of COVID-19, it has now become customary to screen body temperature before entering restaurants or buildings. This is because even 1 °C increase in body temperature is considered dangerous to one's health. Then what if the Earth's temperature rises by 1 °C or 2 °C? For the first time, a POSTECH research team has quantified the length of summer as the Earth's temperature rises.
A POSTECH research team led by Professor Seung-Ki Min and Ph.D. candidate Bo-Joung Park (Division of Environmental Science and Engineering) has examined the lengthening of summer season under 1.5 °C and 2 °C global warming conditions using large-ensemble simulations from multiple global climate models.
1.5 °C and 2 °C are the target temperatures set by the Paris Agreement to mitigate global warming. The goal is to hold the global average temperature rise to 2 °C above pre-industrial conditions and to further limit it to under 1.5 °C. Currently, the global average temperature has already increased by 1.1 °C above pre-industrial conditions.
A difference of half a degree may seem small, but its consequence is enormous. If the global temperature rises by 2 °C, the sea level is predicted to rise by 10 cm compared to 1.5 °C warming conditions. If the warming plateaus at 1.5 °C, close to 10 million people will be freed from the threat the rising sea level. 2 °C warming is also expected to raise the number of people affected by water shortage by up to 50%.
The change in seasonal cycles – which depends on the degree of global warming – is also a factor that greatly impacts many aspects, including agriculture, water resources, and energy of each country. Naturally, when the earth heats up, interest also rises on how long summer will be in which region: But a systematic prediction has been insufficient until now.
To this, Professor Min's research team analyzed the length of summer according to the global warming conditions in the Northern Hemisphere (NH) land areas where an annual cycle is clearly observed. The researchers found that a 2 °C warming would lengthen summer by 20 to 21 days compared to the current 91 days (the warmest quarter of the year in each region), resulting in 111 to 112 days of summer in the mid-latitude regions such as East Asia, the Mediterranean, and USA. Limiting the warming to 1.5 °C reduces the summer season lengthening by 12 to 13 days (Figure 1).
As summer lasts longer, early and late warmings have also become more frequent. Analyzing the frequency of summer-like hot days during the expanded period of future summer seasons (number of days exceeding the average summer temperature of the current climate), their frequency, which is about two days annually in the mid-latitude regions, is predicted to increase to about six days – three times the current rate. That number is brought down to about four days if the warming is reduced to 1.5 °C (Figure 2).
"We have confirmed the lengthening of summer season and the associated increase of frequencies of summer-like days across NH regions according to the Paris Agreement temperature targets," explained Professor Seung-Ki Min. He added, "Mid-latitude lands including East Asia, USA and Mediterranean have been identified as higher risk areas, and it is urgent to assess the impact of such continuous summer expansion on public health, energy, and agriculture sectors and to prepare relevant adaptation measures."
The findings from the study were recently published in the world-renown journal Environmental Research Letters. The study was conducted with the support from the Mid-career Researcher Program and the Science Research Center (SRC) program of the National Research Foundation of Korea.