Yet the great test of model accuracy in predicting human influence on climate is to identify a predicted change in observational data. Klaus turned to the question of what it would take to detect warming in response to increasing greenhouse gases in the context of pronounced climate variability at all time scales his model predicted. statistical. Rather than looking for a needle in the haystack of multidimensional climate variability, the approach he proposed was to search observational data for the pattern of change simulated by climate models in response to atmospheric gases. greenhouse effect that have been robustly predicted by climate models. . The key to success is to identify and then focus on those aspects of the model that are most distinct from natural climate variability, called the “optimal footprint” of climate change.9. It is a noise reduction metric based on the inverse covariance matrix of climate variability.
Under the guidance of Klaus Hasselmann and Hans von Storch, I applied the method to observe trends in global surface temperature data, following Klaus’ argument that one of the expected distinguishing features in the response to increasing greenhouse gases is a fast warming trendten. In parallel, Ben Santer applied an approach closely related to radiosonde and satellite data of the vertical profile of atmospheric temperature change. Based on early modeling work by Suki and signal-to-noise studies from Ben’s collaboration with Klaus11 the vertical profile was expected to show a strong signal of combined tropospheric warming and stratospheric cooling.
In the 1990s, data on surface temperature and atmospheric temperature profiles indicated that the climate was indeed changing in a statistically significant way: they showed a change that exceeded the estimates of unforced or “internal” climate variability that were available. at the time. Since then, these results have been confirmed and reinforced: the observed human-induced change has become stronger, and the models and observations themselves have improved. The divergence of observed changes in climate and internal climate variability now exceeds the statistical threshold that is used for the detection of elementary particles in physics12. Identification of anthropogenic warming is now ‘unequivocal’8. Klaus further proposed a method to explicitly distinguish between the different possible causes of climate change13which we applied to show that the observed warming is consistent with the combined effect of greenhouse gases and aerosols, but cannot be explained by solar variability or greenhouse gas effects without aerosols14.
In addition to major research groups that have developed climate models, Klaus and Suki have used these models to answer fundamental questions about how the climate system works, how it varies, how it changes over time, and how the increase greenhouse gases can affect it. This era of early climate modeling was marked by a host of exciting new discoveries about the mechanisms of climate variability and the causes of the observed changes. Paleoclimatic studies used models to explain the past, and past data was increasingly used to evaluate models. At MPI-M, the intellectual atmosphere was one of exciting and pioneering science where new insights emerged from the application of new modeling tools to understand climate variability and change (see, for example, Fig. 2). We looked at all timescales, from the possible effects of significant changes in ocean patterns that have occurred over thousands to millions of years, going back through Earth’s history, to the future consequences of increase in greenhouse gases over the next few decades, if not centuries.
At MPI-M, the founding director gave direction to science and asked fascinating questions. The group then tried to answer these questions, challenged and led by Klaus, in a golden era of discoveries that those involved remember fondly (see this story15). Talking to people who worked at the GFDL, I had the impression that the scientific atmosphere was similar there. Both Klaus and Suki have been recognized as truly outstanding scientists by their colleagues and colleagues, as well as impressive human beings and interesting people to be around: enthusiastic, cheerful and optimistic.
Klaus remained optimistic not only about scientific research, but also about humanity’s ability to cope with climate change. I sincerely hope that his optimism is well placed and that the move away from fossil fuels will bring new opportunities. Klaus has always advocated trying new things in science too.
In the future
Climate science continues. We do not yet fully understand or predict all of the processes involved in the interactions of the atmosphere, ocean, land surface and biosphere. These interactions are particularly important in terms of the carbon cycle. Considering societal interactions with the Earth system is even trickier, but crucially important. Many climate issues, such as water availability, are fundamentally changed by human actions. Our choices in terms of water management, irrigation, planting or removal of vegetation have implications for the climate. As climate change unfolds, stronger evaporation can fuel droughts and increase extreme heat, and increase fire activity, and then in turn changing vegetation can affect our ability to slow emissions. Melting glaciers and large ice caps influence the climate around them, but also have ramifications for sea level rise and water availability during the summer. There are nonlinearities and tipping points that are best understood and avoided (see https://www.wcrp-climate.org/safe-landing-climates). We also need reliable information to make decisions on how to adapt to climate change.
The two new Nobel laureates guided us through the early days of climate research with great success and opened up a vast field of scientific investigation. Climate science will continue to provide both exciting discoveries about how the planet works and actionable insights into the human footprint on the planet.