Climate change and the carbon cycle David Schimel National Center for Atmospheric Research Boulder Colorado

Indicators of the Human Influence on the Atmosphere during the Industrial Era

Climate change The climate is changing, the climate has always been changing and we are now accelerating the process into uncharted territory

SPM 1b Variations of the Earth’s surface temperature for the past 1,000 years

SPM 1a Variations of the Earth’s surface temperature for the past 140 years

These line plots are misleading by suggesting climate changes uniformly everywhere-change tends to occur non-uniformly in time and space

Percent of the continental USA with a much above normal proportion of total annual precipitation from 1-day extreme events (more than 2 inches or 50.8mm) Karl et al BW 7

The carbon cycle

Fossil fuels are not naturally a part of the fast cycle: every ton emitted changes the carbon cycle for thousands of years

Global patterns of land and ocean uptake

Key point: Most uptake is occurring in the disturbed and managed ecosystems of the Northern Mid-latitudes where 5-20% of plant growth is being stored

Carbon emissions and uptakes since 1800 (Gt C) The biosphere buys time

Measuring carbon uptake

Carbon uptake in the US

Conclusions

Most of today’s carbon uptake is due to historical land use changes: this will change in the future Carbon exchange is sensitive to climate, and especially to growing season length changes Much of the US’s uptake is in montane environments and, in the West, is linked to fire suppression and recovery of forests from historical harvest Carbon management in the US West is linked to watershed management

Land management activities can play a critical role in limiting the build-up of carbon dioxide in the atmosphere, especially in the near-term To stabilize the atmospheric concentration of carbon dioxide (Article 2 of the Convention) will require significant emissions reductions globally, which can only be achieved by either reducing energy emissions or by capture and storage of energy emissions

Key Messages –Human activities (fossil fuel use and land-use) perturb the carbon cycle - - increasing the atmospheric concentration of carbon dioxide –The current terrestrial carbon sink is caused by land management practices, higher carbon dioxide, nitrogen deposition and possibly recent changes in climate –This uptake by the terrestrial biosphere will not continue indefinitely. The question is when will this slow down, stop or even become a source? –Land management results in the sequestration of carbon in three main pools -- above and below ground biomass and soils –Monitoring systems can be put in place to monitor all three pools of carbon –Land management buys time to transform energy systems to lower GHG emitting systems, but will allow more fossil carbon to transferred to the more labile biological pools, hence avoiding a tonne of carbon emissions is better than creating a tonne of sinks