by: Laura Carrea
‘Climate change’, and ‘global warming’: these have been two of the most referenced terms in the media in the past few years. These words sometimes generate controversy and discussion not only on social media or between friends, but also among politicians. Those that do not support climate change measures often state that they “do not believe in it”.
In order to form an opinion on what to ‘believe’ it is crucial to refer to factual evidence from the natural world. Measurements are a clear and objective reflection of what happens in nature and, by definition, they should not create debates. You would be unlikely to hear discussions on whether the temperature this morning was 15°C if it was measured with a reliable thermometer.
My work is on measuring temperatures of inland water, which means lakes, reservoirs, lagoons and rivers, and I try my best to cover lakes all around the world in the most diverse environments. Currently, I am monitoring more than 1,000 lakes throughout the world.
How do I measure temperatures for such a wide range of lakes?
I mainly use satellites, but also the kindness of many people and institutions around the world who measure the temperature of the water directly on the lake.
Satellites are a “modern” way to record temperatures of the whole globe over a day or so. However, they cannot ‘see’ through clouds, leaving some very cloudy areas unrecorded. The satellites I am using have been designed and launched by the European Space Agency (ESA) and by the National Oceanic and Atmospheric Administration (NOAA) with the purpose of monitoring the temperature of the sea and, in general, the surface of our planet.
Given the measurements, my question is: ‘Is the temperature of the water in lakes, reservoirs, lagoons and rivers increasing or decreasing?’
First, I calculated what is called a climatology – a 20-year average of the data (in this case, for 1996 to 2015) of the temperatures for each lake I have investigated. This average value constitutes what it is called ‘the reference’ or ‘baseline’.
I have then collated temperature measurements from 1995 until 2019 and calculated the difference between the temperature of the lake and the climatology during the hottest months. In this way, I have a precise idea whether for example in 2016 a particular lake was warmer or cooler than the 20-year average. This difference is called an anomaly.
I have studied more than 900 lakes across the globe, of which 127 were in Europe. I report here some of my findings for the year 2018 (Carrea L. et al., 2019), but I contribute every year to reports on the state of the climate.
I have looked at the 2018 anomalies for all the lakes I have studied and for the European lakes only. Figure 1 below shows coloured circles in locations that are approximately the position of the lakes I have investigated on the globe. For some areas, like North America and Tibet, the dots in the figure are aligned rather than overlapped. This is a way to display all the lakes in areas where the density of the lakes is very high. A blue dot indicates a negative anomaly, which means that the lake was cooler than the 20-year average while a red dot indicates a positive anomaly which means that the lake was warmer.
This is what I have found:
- In 2018, lakes in Europe, Tibet, New Zealand and mainly the east of the United States were warmer than the 20-year average while the rest of the lakes were cooler or had similar temperature to the 20-year average.
- In 2018 in Europe, the vast majority of lakes were warmer than the 20-year average where the warmest lake was in Germany (lake Constance being 1.68°C warmer than average) and the least warm lake was in Iceland (0.66°C cooler than average).
Figure 1: 2018 lake average temperature anomalies in the warm season. Source: Carrea et al. (2019)
To understand the thermal behaviour of the lakes through the years, I have plotted for each year the value of the anomaly for the European lakes only, and for all the lakes together. The plot can be seen in Figure 2 and from it we can deduce the following:
- In 2018 the European lakes were overall 0.83°C above the 20-year average. This is the highest value since 1995 at least.
- Putting together all the lakes I have investigated (European and non-European), overall in 2018 they were 0.17°C above the 20-year average.
- Excluding the European lakes, I found that the rest (800 lakes) were only 0.06°C above the 20-year average, which is much less than the anomaly for European lakes.
- Since 1995 the overall temperature of the lakes is increasing, and I have found that the European lakes are warming faster than all lakes globally.
Figure 2: Average lake surface water temperature anomalies per year for 923 lakes worldwide and or 127 European lakes. Source: European State of the Climate in 2018.
For Europe, 2018 was a very hot year, which was also confirmed by other sources of data (Toreti et al. 2019), but clearly, lakes are warming and some of them at a very worrying pace.
What is the problem with warming lakes? The temperature of the water has a strong impact on the lake ecosystem and can, among other things, lead to an increase in the development of toxic cyanobacteria blooms. The resulting poor water quality endangers the ecosystem and therefore the life that it supports, including human life.