Greenhouse gases like carbon dioxide trap heat in the atmosphere and are causing global temperatures to rise at the fastest rate in recorded history. The 10 hottest years on record have all occurred since 1990. If emissions are not curbed, average US temperatures could increase by 3 to 9 degrees by 2100. Observable changes include rising sea and air temperatures, melting ice and snow, and rising sea levels. Various ecosystems and species are already being negatively impacted by the climate changes. To mitigate these effects, emissions of greenhouse gases must be reduced and alternative energies developed.