In this lesson, we examined some of the key anthropogenic climate change projections. Some of the main findings were:
- climate models project anywhere from 0.2 to 7°C warming over the next century, depending on two critical variables: (1) what decisions society makes regarding future carbon emissions and (2) currently irreducible uncertainties regarding the sensitivity of the climate to greenhouse gas radiative forcing;
- the primary source of spread in projected warming is factor #1, the uncertainty in future emissions. Were we to freeze greenhouse gases at their current levels, the average projection among models is an additional warming of 0.5°C. This scenario is highly unlikely, as it is very difficult to find a pathway to zero emissions in the near future. On the other hand, were we to pursue business-as-usual emissions scenario (e.g., continue on the A1FI scenario), the average projection among models is for an additional warming of 4°C;
- the impact of factor #2, the uncertainty in climate sensitivity, is nonetheless quite significant. In the A1FI scenario, the globe could warm anywhere from a lower bound of 2.5°C to an upper bound of 6.5°C, depending on which particular climate model is used;
- the variability in temperatures in both space and time is projected to be considerable. High latitudes warm more than low latitudes owing to positive feedbacks related to the melting of ice, and land warms more than oceans due in large part to the greater thermal inertia of the oceans. Even as the globe warms, there will continue to be cold periods over particular regions related to ENSO and other sources of natural variability;
- precipitation is projected to increase in the tropics and sub-polar latitudes, while decreases are projected for sub-tropical through mid-latitude regions. These changes reflect a combination of the effects of shifting storm tracks and the potential for a warmer atmosphere to hold more water vapor;
- continental drought becomes more widespread over much of the continents. This results from a tendency for increased evaporation to dry out soil, even in many regions that see an increase in precipitation;
- anthropogenic climate change leads to substantial changes in atmospheric circulation, including a poleward shift of the descending branch of the Hadley Circulation and of the jet streams, polar front, and storm tracks. These changes also include a weakening of monsoonal circulations, and possible, but uncertain, impacts on the Walker Circulation pattern associated with ENSO;
- the NAO/AO/NAM mode of variability, tied with variations in the position of the Northern Hemisphere winter jet stream, is projected to become more positive, associated with a northward displaced storm track and warmer, wetter winter conditions in regions such as Europe, but drier conditions in semi-arid regions such as the Mediterranean and Near and Middle East;
- a modest weakening is projected for the meridional overturning ocean circulation (variously referred to as the thermohaline circulation and conveyor belt circulation). State-of-the-art models, however, project a far weaker effect than what was once considered possible; rather than leading to cold conditions over the North Atlantic and neighboring regions, what is currently projected is only a moderate decrease of the warming in a small region in the North Atlantic ocean south of Greenland;
- projected changes in the character of the El Niño/Southern Oscillation are uncertain. Model projections are divided with respect to whether the future climate state will be more like El Niño or La Niña, and whether individual El Niño and La Niña events will be larger or smaller.
Reminder - Complete all of the lesson tasks!
You have finished Lesson 7. Double-check the list of requirements on the first page of this lesson to make sure you have completed all of the activities listed there before beginning the next lesson.