The temperature curve for the Baltic Sea region shows a clear trend. It indicates a rise by up to two degrees Celsius over the past thirty years. How did this happen? Looking for clues, climate researchers must focus on the recent past. In doing so, we proceed just like criminal investigators facing a tricky case: The wealthy Countess Celsius is lying dead on the floor. Did she die of natural causes or was she murdered? Similarly, climate change detectives will ask if the Baltic Sea area’s temperature increase is due to natural fluctuations or external factors.
In collaboration with my colleagues from the Cluster of Excellence CliSAP, I start this climate murder investigation by analyzing all temperature measurements since 1980. We document the scene and collect potential evidence. If our data can merely be attributed to regular variations, we do not have a case, meaning that the Countess simply died from decrepitude. But as it turns out, the summer and fall as well as the average annual temperatures are freak values calling for clarification. Hence, we post a “Most Wanted” notice.
A prime suspect has already been identified. The relentless rise of the greenhouse gas carbon dioxide (CO2) may explain the warming effect throughout the year. But how do we prove this? Computer-generated climate models can lead us onto the right track. These simulations provide suggestions as to what increases in temperature may derive from higher CO2 levels. In the current case our modeling reveals that—regarding the winter and spring months—CO2 may indeed be the sole culprit.
Nonetheless, we cannot close our case yet. The established rise in summer and fall temperatures since 1980 is too high. According to CO2-driven computer models it ought to be lower; ergo we must conclude that CO2 cannot be held responsible alone. It still remains a mystery what other factors contributed to those unusually warm months.
For that reason, we go on tracing further evidence. We suspect that tiny dust and dirt particles in the air may also play a major role. These so-called aerosols reflect the sunlight and block out incoming light like sunshades, thus cooling the bottom layers of the Earth’s surface. Moreover, the particles facilitate the formation of clouds—a way of cooling by obscuring. So, aerosols act as a brake on temperature. Presumably, this mechanism is particularly efficient in summer and autumn.
In the course of the massive industrialization of the Baltic Sea region, the aerosol concentration in the air had increased significantly. We can therefore deduce that up until the 1980s, aerosols mitigated the warming effect in the Baltic area. Apparently, the pollution of the environment reduced regional climate change impacts for a while. Things changed, however, when the clean air policy became effective in the eighties. As manufactured aerosols became less used, their summer and fall cooling effects decreased as well. This revelation demystifies the immense rise in temperatures over the past decades.
Our murder case seems solved now. Carbon dioxide has been found guilty of warming the temperature in the Baltic region. We proved the following: until the 1980s, Countess Celsius could still rely on servants that, at first glance, appeared ill-suited to compensate any temperature-related predicament. Rather paradoxically, abundant aerosols alleviated our fragile lady’s heat exhaustion by cooling the air, especially in the summer and fall. As soon as they started diminishing, she grew increasingly defenseless…