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Climate Sensitivity on Decadal to Century Timescales: Implications for Civilization

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Session Description: 

There is growing awareness that feedback processes in Earth’s climate system operate on multiple timescales. So-called fast feedback processes work on time scales of days to decades and include feedbacks due to changes in water vapor, clouds, aerosols, and sea-ice, to name a few. However, paleoclimate data indicate that Earth’s climate system becomes more sensitive to increases in greenhouse gases on longer timescales, i.e. centuries to millennia. These feedbacks include changes to vegetation, glacial ice fields, high latitude carbon pools and potential reservoirs of methane on ocean shelves.

It is projected that if we continue on a business-as-usual course for carbon emissions, then by 2100 atmospheric carbon dioxide levels will reach 800 to 1100 ppmv (see IPCC, AR4). The last time these levels of CO2 existed in the atmosphere was around 30 to 40 million years ago. While it took millions of years to reach these levels of CO2 in Earth’s past through natural processes, the human burning of fossil fuels will return Earth to these elevated levels in little over a century. Thus, the present and near future rate of change in greenhouse gas forcing of the climate system is unprecedented. Earth’s past record of warming also indicates the presence of slow feedback processes that may amplify the climate sensitivity beyond the canonical value of 1.5 to 4.5 °C warming due to a doubling of CO2. Studies by Hansen et al. (2008), Lunt et al. (2010) and Kiehl (2011) find this canonical value is roughly twice as large on longer time scales. Thus, we are looking at climate sensitivities on the order of 6 to 8 °C for a doubling of CO2 on the century to millennia time frame.

The implications of this heightened sensitivity for human civilization have not been explored to date. Studies on adaptation and mitigation have focused on at most the time frame of the next few decades to the end of this century. Yet, geochemical modeling clearly indicates that once CO2 levels reach ~800 ppmv, humanity has committed Earth to existing with elevated levels of CO2 for the next 50,000 to 100,000 years. In terms of societies this has been termed the irreversible fate of Earth’s climate. It is notable that past transient warm climate events, such as the Paleocene Eocene Thermal Maximum (PETM) that took place ~55 Ma confirm the existence of long residence times for elevated atmospheric CO2.

This workshop will address the scientific and societal implications of Earth’s fast and slow feedback processes. The foci will be on investigating the role of slow feedback process on climate change over the next few centuries and how human civilization will need to transform if we are to avoid the implications of large climate change. Some questions to be explored include: How resilient are societies to amplified climate change on multiple time scales, with system surprises and unexpected tipping points? How does the biosphere – its ecosystems and services --respond to amplified warming beyond what is expected over the next few centuries? Are there tipping points to global ecosystems? What social transformations are required to avoid the catastrophic consequences of large climate change? These are critical questions that need exploring given that current carbon emissions now exceed the upper limits of IPCC business-as-usual scenarios.

In the early part of workshop, paleoclimate scientists will provide a view of what Earth was like under elevated atmospheric CO2 in the past, including the hydrological cycle, biogeochemical cycles and life on Earth. Climate modelers will provide information on the latest developments of including processes involving glacial ice models, methane hydrate models and vegetation models as new feedbacks to the climate system. The mid part of the workshop will integrate this understanding of feedbacks on various time scales with projected climate change over the coming centuries, and consider how these climate changes may effect civilization and global ecosystems. The last two days of the workshop will focus on what transformations – social, economic, political, cultural and behavioral – are required to avoid catastrophic climate disruption.

The barriers to transformation, including psychological and emotional factors, will also be considered. Drawing upon diverse perspectives and approaches, this part of the workshop will emphasize innovative and creative responses to climate change that take into consideration the long-term implications of fast and slow feedbacks. We will consider the challenges posed by our current paradigms (e.g., social and economic paradigms, worldviews and belief systems, etc.). The participants for this part of the workshop will include representatives from the social sciences and humanities.

The interdisciplinary discussions at the workshop will help further the development of the next generation of Earth system models for applications on climate change on timescales of centuries to millennia, as well as help to improve communication of the long-term implications of greenhouse gas emissions to the public and policymakers. Given these timescales and the current integration of integrated assessment components into Earth system models, the workshop comes at a critical point in the development of more comprehensive models to consider the implications of long-term climate change on society and ecosystems.

The workshop also affords an opportunity to hold discussions on creating transformative solutions to our business-as-usual world system. The investigation of long-term solutions is critical at this point in the history of civilization given that much of the current policy discussions are focused on the present to 2050 time horizon. Given the long lifetime of atmospheric CO2 (centuries to millennia), we must begin to consider various ways of social transformation that can address the long-term implications of climate change, especially given the magnitude of this change anticipated under elevated CO2. This includes not only reducing greenhouse gas emissions, but also addressing vulnerability, disaster risk reduction and adaptation to climate variability and change.

The outcome of the workshop will be a review paper summarizing the findings of the workshop with recommendations for future research into long-term climate change under enhanced warming. We will also discuss with participants the possibility of a special journal issue presenting the specific findings of the workshop.