Is the science too uncertain for us to be able to make any sensible decisions about the future?
Certain effects may have a strong influence on local or regional climate; the details are hard to predict accurately. Scientists are much more confident of the broader global warming trend.
Ocean currents are driven, amongst other factors, by water temperature and salinity (saltiness). In the northern North Atlantic water increases in density, due to winter cooling of the water, and salinity increases, due to evaporation from the surface - both effects cause the water to sink. This drives the ocean currents in the Atlantic Ocean. However, melting land ice, which dilutes the North Atlantic waters and makes them less dense, lowers the density of this North Atlantic water so reducing its ability to sink. Thus global warming may be capable of disrupting the Atlantic current system. Although the possible mechanisms of disruption are understood, the precise changes in the currents are difficult to predict with much confidence.
There is evidence to suggest that the Atlantic current system may have shut down when a huge volume of freshwater, derived from melting of the North American ice sheet at the end of an Ice Age, was suddenly released by the rupture of a natural dam. So, in 2005, climatologists were shocked by data that suggested that disruption of Atlantic currents might already be happening. However, subsequent measurements showed no clear trend. Relatively few scientists think there will be a rapid shutdown of ocean currents since most ocean models predict no more than a slowdown, probably towards the end of the 21st century. This could locally slow, or even reverse, some of the warming due to greenhouse gases emitted by mankind, which might even be welcome in an overheated Europe. Ultimately, both in the long run and on a global scale, local effects such as this will be insignificant compared with the much greater changes caused directly by global warming.
Scientific reports written in the 1970s suggested that cooling from industrial dust particles in the atmosphere might outweigh the effects of global warming. These were given disproportionate media attention, given that between 1965 and 1979, 44 scientific publications predicted warming, 20 were neutral and just 7 predicted cooling. Since that time there has been less particulate pollution and any cooling effect is much less marked.
There is always a possibility that certain effects will have a strong influence on local or regional climate and the details of these are indeed more difficult to predict accurately. However scientists are much more confident of the broader global trends.