twentieth century is disturbing and suggests that there could be large errors in current estimates of natural ozone levels and the sensitivity of ozone to human influence. In the troposphere these errors could relate to emissions of precursors, heterogeneous and homogeneous chemical processes, and stratospheric influence. Lightning emissions of nitrogen oxides (NO_x) are particularly uncertain and yet play a major role for ozone production in the middle and upper troposphere where the radiative effect is maximum. Transport of ozone between the stratosphere and troposphere greatly affects upper tropospheric concentrations in a manner that is still poorly understood.

RECOMMENDATIONS:

• Improve knowledge of sources of ozone precursors, including in particular nitrogen oxide emissions from lightning.

Improve understanding of the transport of ozone in the upper troposphere and lower stratosphere region and the ability of models to describe this transport.

• Use observed long-term century trends in ozone concentrations to evaluate and improve global chemical transport models.

Paleoclimate records indicate that climate can change so rapidly and unexpectedly that human or natural systems may have difficulty adapting. Such abrupt climate changes take place when “the climate system is forced to cross some threshold, triggering a transition to a new state at a rate determined by the climate system itself and faster than the cause” (NRC, 2002). The Earth’s climate has experienced abrupt shifts in temperature and precipitation during the preindustrial Holocene. Each of these events appears to have weakened the North Atlantic thermohaline circulation enough to cause abrupt cooling of the northern North Atlantic, Greenland, Iceland, and Europe.

The present climate could undergo abrupt changes in the future, not necessarily by the same mechanisms as in the past. Models imply, for example, that greenhouse warming may alter the hydrologic cycle enough to freshen North Atlantic surface waters and shift thermohaline circulation closer to a threshold. Collapse of parts of the Greenland ice sheet could be a risk factor as suggested by evidence that meltwater-induced basal sliding of southern parts of the ice sheet toward the ocean may have begun within the last decade. Knowledge of what triggers abrupt climate changes is still quite limited; more research is needed to determine the possible role of radiative and nonradiative climate forcings, such as human-caused increases in greenhouse gases or land-use changes. Indeed, past abrupt climate