Aside from the actual crater, a heat pulse and shock wave will affect the immediate environment. Large impacts will have regional, or even global effects that may kill off many organisms, or even cause mass extinctions of species. Re-entering ejecta overheat the atmosphere causing forest fires, ozone depletion, possibly greenhouse warming depending on the composition of the target, followed by ‘nuclear winter’ cooling caused by material lofted into the stratosphere. Impacts into ocean present an additional danger. Impact-generated tsunamis could devastate huge areas, and since oceans are large targets, this may represent the most substantial impact-related hazard to human populations.