The controversy between the IPCC and Non-governmental IPCC(NIPCC) on the attribution of global warming are reviewed.IPCC holds that today's global warming is mainly due to anthropogenic activities rather than natural variability,which is emphasized by NIPCC.The surface temperature observations since the mid-20th century support the hypothesis of anthropogenic impact,but for the last one hundred years or so,natural forcings such as solar activity, volcanic eruptions and thermohaline circulation variations also have had great influences on the Earth's climate,especially on inter-decadal timescales.In addition,evidence suggests that the Medieval Warm Period(MWP) and Little Ice Age (LIA) are closely associated with the solar activity over the past 1 thousand years.Over the past 10 thousand years, the North Atlantic cold events and solar activity are closely correlated.Nevertheless,the physical mechanisms of the solar-climate variability and interrelation are not well understood,yet.Notably,a prevailing view recently indicates that galactic cosmic rays may result in climatic cooling through modulating global low cloud cover.However,its process and mechanism need to be further investigated.
Global warming is unequivocal,as is manifested in three internationally known global temperature records with the global average temperature rising at a rate of 0.70-0.75℃ per 100 years during 1910-2009 [Wen et al.,2011].In other words,the earth has warmed more than 0.8℃ since the Industrial Revolution.If it is true that 2℃ above pre-industrial level is a threshold of climate "safety",we will have to confront a serious situation where less than 1.2℃ warming is permissible in the future [Liu et al.,2011].According to climate system concept [Wang,2011],the global warming is not only reflected in global temperature change but also in changes of other climate system components,such as widespread melting of snow and ice,rising of global average sea level,and permafrost degradation [IPCC,2007].
