The effects of solar activity shows up in two aspects: firstly, changes in the earth’s orbital parameters that affect climate for tens or even hundreds of thousands of years. Because of the changing parameters, the relative positions between the earth and sun are changing. Although only slight changes are detected in total solar radiation reaching the earth, great changes happens in the distribution of surface radiation at different latitudes and seasons, which would induce great climate changes in the Northern Hemisphere and globally. Secondly, the sun experiences periodic or aperiodic active periods when it produces more sunspots with stronger radiation, intense magnetic activity and more high-energy particles, which is called solar activity. The most evident solar cycle is about 11 years. Many scientists believe that the earth would become warmer or cooler when sunspots increase or decrease in number. And so far, scientists have ruled out that changes in solar radiation are the main cause of global warming.

The dust veil after volcanic eruptions has an important impact on climate. It can spread across the entire hemisphere, the greatest impacts occurring in the middle and high latitudes. The lifetime of volcanic ash particles lasts for 3 to 7 years in general, even up to 15 years. If volcanic eruptions occur frequently, the cumulative effect of dust veil would last for more than 100 years. Taking the volcanic eruption in Iceland this year as an example, its impacts can not be well known in short time. Volcanic dust veil affects atmospheric transparency, leading to the decrease of direct radiation and increase of scattered radiation, with a certain decrease in total radiation, which can cause lower temperature globally. Because of smaller solar altitude angle, the higher the latitude is, the more evident the decreasing trend of temperature is. Precipitation would also increase after volcanic eruption. For the global temperature change beyond decadal scale, the precise impacts of volcanic activities on global air temperature over the past 100 years are not yet clear, because there are no reliable time sequences to represent global volcanic eruptions and stratospheric volcanic dust veil.

Shortwave solar radiation is able to pass through atmospheric greenhouse gases (GHGs), such as carbon dioxide, methane, nitrous oxide and HCFCs, but outgoing long-wave radiation is blocked, resulting in the warming of the atmosphere. A certain concentration of GHGs can absorb thermal energy from the earth’s outgoing radiation and reflect it back to the surface, raising surface temperatures even higher. Generally, scientists believe that the enhanced “greenhouse effect” is probably the main reason for the evident increase in global surface temperature over the last 100 years.

Besides the enhanced greenhouse effect caused by GHG emissions, human activities release CFCs, Halons and HCFCs into the atmosphere, leading to stratospheric ozone depletion that also contributes to climate change. Human activities also change surface albedo through land-use change to affect climate (it mainly appears in mid-latitude land areas in summer). However, these effects are relatively small compared with greenhouse effect.

Currently, the increasing rate of GHG concentration is well consistent with that of emissions due to human activities, which is unprecedented in the atmosphere over the last several thousand years. Furthermore, changes in carbon isotope ratio and atmospheric CO2 distribution are also consistent with human emissions, which have proved that human activities contribute to the increase of GHG concentration. The rapid increase of GHG concentration in the past 100 years has been unprecedented in the last 420,000 years, or probably in the last 20 million years. Additionally, carbon budget models quite accurately reconstruct global carbon cycle process, which also indicates the effect of human activity. The similar studies on methane and nitrous oxide show that human activity is the main source of GHGs.

The bubble samples collected from paleo-polar ice cores in Greenland and Antarctica clearly indicate that in the current interglacial period, atmospheric CO2 concentration over 10,000 years before industrialization remains 280 ppmv with amplitude of only a few percentages. It suggests that natural carbon budget during this period is balanced, i.e. the average inflow is equal to the outflow. The facts and evidences from other sources show that the accumulation of carbon budget imbalance as a result of human activity is the main reason behind the increase of CO2 concentration by 31 percent in the last several centuries.

Fossil fuel use contributes 70-90 percent of total CO2 emissions from human activities. Fossil fuels are used for transportation, industrial production, heating, cooling, power generation, etc. The rest of CO2 emissions are from land use activities, such as animal husbandry, agriculture, forest clearing and degradation. Other main sources of GHGs include the use of fossil fuels in production and transportation, agricultural activity, waste management and industrial processes.

However, because of the uncertainty in biogeochemistry processes involved by natural and anthropogenic emissions, it is difficult to well understand the exact contributions of human activity to GHG emissions.

In order to prove that climate change over the last 50 years is likely due to human activities, scientists have compared the simulations of climate models with observations in the last 100 years. It is found that if only considering natural fluctuations in climate or the impact of human activity, it is hard to well simulate past climate change. However, if both are considered together, the climate change over the last 100 years would be well simulated, which proves that human activities are the main reason for the global climate change over the last 50 years.

In particular, it should point out that during the period of constant temperature increase, the economic activity in the course of industrialization in developed countries has been the main reason for global climate change due to a great deal of fossil fuel consumption, deforestation in tropics, and production and use of synthetic chemical products, which led to a large amount of GHG emissions. According to the reports from relevant institutions, since 1750, accumulative CO2 emissions in the world have reached a total of one trillion tons, of which 80 percent came from developed countries. Even now, developed countries still maintain a level of per capita emissions that is far higher than that of developing countries, and most of their emissions are attributed to consumption. In comparison, emissions from developing countries are primarily survival emissions and international transfer emissions. Through the comparison of statistics, it can be clearly seen. For the period of 1850-2005, China’s total emissions has accounted for only 8.28 percent of the world’s total emissions, while 28.3 percent for the U.S.; the contribution of per capita historical cumulative emissions of China to climate change has been only one percent, far less than 21.3 percent of the U.S. and 16.4 percent of the UK. In 2006, China’s per capita emissions were 4.32 tons, higher than the world’s average (4.18 tons), but only one fifth of the U.S. emissions.

Currently, China is at the key stage of modernization construction. A peaceful and stable international environment is vital to sustaining better and faster economic and social development. Climate change is a major global issue. Adaptation and mitigation of climate change require joint endeavors by the international community. China should envisage its current development stage and urgent needs for development, and be fully aware of pressures and challenges it is facing, such as a large amount of GHG emissions and international calls for emission reduction in China. In this regard, China should actively participate in international activities in response to climate change to promote global and regional cooperation in politics, economy, science and technology based on the principles of mutual benefit, win-win progress and in support of development. It is necessary to practically improve scientific and technological levels in prediction, projection and impact assessment of global and regional climate change, to formulate strategies for climate change adaptation, and to improve China’s scientific and technological capacity in tackling climate change.