However, the biggest test of the models ability to predict human influence on climate is to detect a predicted shift in observational data. Klaus asked the question: What would it take to detect warming due to rising greenhouse gases against the backdrop of climate variability on all timescales that his model predicted. Instead of trying to find a single piece of information in the maze of multi-dimensional climate variability and uncertainty, Klaus suggested that we search observational data for the pattern of climate-model-simulated changes in response to greenhouse gasses that have been well predicted by climate models. The key to success is to find and then focus only on those aspects that are distinct from natural climate variability. This is known as the optimal fingerprint of climate changes.9. This involves a noise-reducing measure based upon the inverse climate variability covariance matrix.
Klaus Hasselmanns’ and Hans von Storchs’ guidance, I applied the method for observing trends in global temperature data. Klaus argued that one of the distinguishing features in the response to greenhouse gases increases is a rapid warming trend.10. Ben Santer used a similar approach to satellite and radiosonde data on the vertical profile atmospheric temperature change. Based on Suki’s early modelling work as well as signal-to-noise research from Bens collaboration avec Klaus11The vertical profile was expected show strong evidence of combined tropospheric heating and stratospheric cool.
Both the surface temperature and the atmospheric temperature profile data had shown that climate was changing statistically in the 1990s. These data were more than the available estimates of the unforced climate variability or the internal climate variability. These results have been confirmed and strengthened over the years: both observations and models have improved, and the observed human-induced changes have become more strong. The statistical threshold for detection of elementary particles in Physics is exceeded by the divergence in observed climate changes and internal climate variability.12. The identification and confirmation of anthropogenic warming are now unambiguous8. Klaus also suggested a method to clearly distinguish between possible causes of climate changes13We used this to show that the observed warming was consistent with the combined effect from greenhouse gases and aerosols. However, it could not be explained either by solar variability or by greenhouse gas effects without Aerosols.14.
Klaus and Suki were both leaders of research groups that developed climate modeling. These models were used by Klaus and Suki to address fundamental questions about the climate system, including how it changes, how it evolves over time, and how increasing greenhouse gas emissions may affect it. There were many new discoveries made in this age of early climate modelling about mechanisms of climate variability, and causes of observed climate change. Models were used in Palaeoclimatic Studies to explain the past. Past data were increasingly used for evaluation of the models. The intellectual atmosphere at MPI-M was one of pioneering science and excitement. New insights were gained by using new modelling tools to understand climate variability, and change (see, for instance, Fig. We examined all timescales from the possible impacts of large changes in the ocean configurations over thousands to many millions of years back in Earth’s past to the future effects of greenhouse gas rises over the coming decades and centuries.
The founding director of MPI-M gave science direction and asked fascinating questions. Klaus challenged and directed the group to try to answer these questions. It was a great time of discovery that all involved remember fondly.15). Talking with people who worked at GFDL gave me the impression that there was a similar scientific atmosphere. Both Klaus, and Suki, were recognized by colleagues and coworkers as truly outstanding scientists. They were also impressive human beings who were fun to be around.
Klaus kept his optimism high about scientific inquiry and humanity’s ability deal with climate change. I believe his optimism is well-placed. I also hope that we will see new opportunities by moving away from fossil energy. Klaus has always advocated for trying new things in science.
The future is now
Climate science is still evolving. We do not yet have a complete understanding or prediction of all processes involved with the interactions of atmosphere and ocean, land surface, biosphere, and biosphere. These interactions are crucial for understanding the carbon cycle. Even more important is the consideration of societal interactions with Earth system. Human actions can have a major impact on climate, such as water availability. Climate can be affected by our choices regarding water management, irrigation and the planting or removing of vegetation. Climate change is likely to accelerate as stronger evaporation can fuel droughts, increase extreme heat, and increase fire activity. Then, a changing vegetation could impact our ability to slow down emissions. The climate around melting glaciers and large-sized ice sheets has ramifications for sea levels rise and water availability during the summer. We need to be aware of nonlinearities and tipping point that we can avoid and better understand. https://www.wcrp-climate.org/safe-landing-climates). Reliable information is also necessary to make informed decisions on how to adapt.
These two Nobel laureates helped us navigate the early days in climate research with great success and opened up a wide field of scientific inquiry. The exciting discoveries in climate science will continue to provide valuable information about the planet’s functioning and practical advice on how humans can make a difference.