As anthropogenic greenhouse gas emissions reduce the stability of the global climate, abrupt climate change could become more intense. The impact of an asteroid or comet larger than about 1 km diameter has the potential to block the sun globally, causing impact winter. Particles in the troposphere would quickly rain out, but particles in the stratosphere, especially sulfate, could remain there for years. Similarly, a supervolcanic eruption would reduce the potential of agricultural production from solar photosynthesis, causing volcanic winter. The Toba super volcanic eruption approximately 70,000 years ago may have nearly caused the extinction of humans (see Toba catastrophe theory). Again, primarily sulfate particles could block the sun for years. Solar blocking is not limited to natural causes as nuclear winter is also possible, which refers to the scenario involving widespread nuclear war and burning of cities that release soot into the stratosphere that would stay there for about 10 years. The high stratospheric temperatures produced by soot absorbing solar radiation would create near-global ozone hole conditions even for a regional nuclear conflict.