Observed Climate Changes and Their Causes

By ZHENG GUOGUANG

ZHENG GUOGUANG is Administrator of the China Meteorological Administration.

Climate change is a major global challenge facing the world today. It relates to the survival of the human beings and the development of all countries, and threatens social and economic development and people's livelihoods. Current research shows that global climate is mainly characterized by warming over the past 100 years; meanwhile, there still remains great uncertainty in the science of climate change.

Observed Changes in Climate and Their Effects

Observations, impacts and future trends of global climate change: Observations indicate that global average surface temperature over the past century (1906 to 2005) increased by 0.74°C, and 0.65°C for the period 1956 to 2005. Eleven years among 1995 to 2006 rank the 12 warmest years since the instrumental record. Average Northern Hemisphere temperature during the second half of the 20th century was likely the highest in the past 1300 years. The snow cover in the Northern Hemisphere significantly shrunk, and mountain glaciers and Greenland ice sheets were melting at an accelerated speed. The thermal expansion of seawater as ocean temperature warms has contributed to the continued rise of global average sea level, and the total sea level rise in the 20th century is estimated to be 0.17m. Radiosonde and satellite measurements of the temperature of the atmospheric layer of 8 to 12 km above the earth (i.e., troposphere) show warming rates that are consistent with those of the surface temperature record. The above-mentioned phenomena have been proved by the observations released by many international research institutes. Global warming over the past century is an objective and unequivocal fact generally recognized and accepted by international and scientific communities.

Global warming has led to more frequent and intense extreme climate events. Since the 1950s, many areas in the world have seen more frequent heat waves and local floods, heavy precipitation and more violent storms. For example, more frequent, intense and longer droughts have been observed in the tropics and subtropics (particularly in Africa) since the 1970s, with wider affected areas.

Global warming has brought certain effects on cryosphere, water resources, ecosystems and coastal areas. Observations show that glaciers have been melting at an accelerated speed, and the earth has experienced an enlargement and increased numbers of glacial lakes; increasing ground instability in permafrost regions and rock avalanches in mountain regions have been observed. Average temperatures in the Arctic have been increasing at almost twice the rate of the rest of the world in the past 100 years, and sea ice extent has shrunk significantly, with a decrease in the thickness of the sea ice in spring up to 40 percent. Permafrost in the Northern Hemisphere is thawing. Increased run-off and earlier spring peak discharge in many glacier- and snow-fed rivers, and warming of lakes and rivers in many regions are occurring. Future conflicts of imbalanced spatial and temporal distributions of water resources will be more prominent. Water resources will decrease in some dry areas and increase in some wet areas. Water storage in glaciers and snow cover will reduce, and drought-affected areas are projected to increase, with potential impacts on available water for more than one-sixth of the world population. Significant changes are taking place in some ecosystems of the Polar Regions. Plant and animal species are inching towards the Polar Regions and high altitude areas. Earlier timing of spring events such as leaf-unfolding, bird migration and egg-laying, resulted in ecological imbalance. The oceans can absorb more carbon dioxide (CO2), which leads to a several-fold increase in surface ocean acidification, thus posing a greater threat to marine ecosystems, especially shellfish. Coastal wetlands and mangroves are vanishing and many coastal regions are battered by severe damages from storm surges. It is projected that the global average sea level will rise between 0.2 to 0.6 m by 2100, which will be a great threat to the socio-economic development and ecological safety in coastal and low-lying areas, especially in the large deltas of Asia (including Yangtze River and Pearl River in China), Africa and some small islands. The frequency of floods, storms, salt tide and other natural disasters in coastal regions may increase due to sea level rise. It is projected that the population affected by floods would be doubled or even tripled by 2080.

The influence of global climate change continues and might even be more acute. According to the Fourth Assessment Report (AR4) of the Intergovernmental Panel on Climate Change (IPCC), the global average surface temperature is projected to increase by 0.4°C in 2020 relative to the last two decades in the 20th century, and 1.1°C to 6.4°C by the end of this century with the most remarkable warming occurring over land and at high latitudes in the Northern Hemisphere. It is projected that precipitation would likely increase in the middle and high latitudes, while decrease in most tropical and subtropical regions. The frequency of high temperatures, heat waves and heavy precipitation events is very likely to continue to increase. Typhoons and hurricanes are expected to be more intense with heavier precipitation and more destruction. Once-in-a-millennium flood may happen once in one hundred years; and once-in-a-century flood may happen once in fifty years or even shorter period. Some regions will experience extreme events that have never happened before. The impact of climate change is a multi-headed monster, simultaneously facing off with cryosphere, agriculture, water resources, ecosystems, coastal areas, human health and other aspects. Future impacts of climate change are expected to be more severe.

Observations, impacts and future trends of China’s climate change: China has been experiencing a warming trend similar to that in the rest of the world. The data released by the National Climate Center of the China Meteorological Administration indicated that the average surface temperature in China rose by 1.1°C between 1908 and 2007. Warming in northern China is more obvious: a rise of 4 °C in last 50 years. The year 2007 was the warmest since 1951. In 2008, annual mean temperature was 0.7°C higher than normal levels, making this year the seventh warmest since 1951, and also the twelfth successive year that temperature is higher than normal. Moreover, the urbanization process started rather late in China. So the urban heat island effect has become very obvious only since the 1980s, thus accelerating this warming trend.

The pattern of precipitation distribution in China has changed significantly with climate warming. In the past 50 years, precipitation in west China has increased by 15 percent to 50 percent; eastern China saw frequent droughts in the north and floods in the south; precipitation in South China increased by 5 percent to 10 percent, while in North China and most regions of Northeast China it decreased by 10 percent to 30 percent. Projections show that annual mean precipitation in China by 2020 would somewhat increase compared to that in the last two decades in the 20th century, 2 percent to 5 percent by 2050, and 6 percent to 14 percent by the end of this century. The magnitude of total precipitation increase in the north will be greater than that in the south. The number of days with heavy rain in the south will increase.

Extreme climate events in China are becoming more frequent and intense. First, high temperatures and heat waves in summer have become more frequent. Especially after 1998, the number of days with high temperatures above 35°C has been continuously and significantly higher than normal average. Second, regional droughts have become more severe. Especially in North China, droughts happened eight years of the last 20 decades, being the worst situation since 1886 either the frequency of droughts or their scope and loss. Third, heavy precipitation events have increased. The last two decades are the flood peak period in the Yangtze and Huaihe rivers following the 1950s, resulting in annual mean direct economic losses up to RMB 125 billion. In the 21st century, extreme high temperature events are projected to be more frequent; the increase in precipitation will be mainly characterized by an increase in heavy precipitation events; and typhoons and severe convective weather are expected to be more intense.

Record-breaking extreme events have frequently occurred in recent years. For example, Chongqing was hit by once-a-century heat wave and drought in 2006. In February 2007, the Chongqing section of the Yangtze River reached the lowest level since hydrological record began in 1892, resulting in the prohibition of shipping for a limited time. In July, Chongqing was again hit by once-a-century heavy rain and flood. “Saomai”, once-a-century super strong typhoon, made landfall in Zhejiang Province in August 2008. No.1 typhoon “Neoguri” in 2008 was the earliest typhoon landing in China since 1949. In 2007, Jinan in Shandong Province was hit by heavy rain, with an hourly rainfall of 151 mm, a historical maximum since 1958. In the same year, a huge flood, second only to 1954, occurred in the Huaihe River Basin. In early 2008, once-in-fifty-year (once-in-a-century in some regions) low temperature, sleet, snow and freezing extreme weather occurred in southern China, and had serious effects on the economic and social development in China. In the winter wheat region of North China, the precipitation has declined significantly since November 2008, thus leading to a widespread meteorological drought that hit 12 provinces (municipalities and autonomous regions) including Beijing, Tianjin, Hebei, Shanxi, Shandong, Henan, Anhui, Jiangsu, Hubei, Shannxi, Gansu and Ningxia. Droughts were once in 30 years in most regions or 50 years in some regions. Since the winter of 2009-2010 in the Northern Hemisphere, cold waves and snow storms swept the southeast of North America, northern Europe and eastern China. Three record-breaking early snow and snow storms in China and recent several cold events in the Northern Hemisphere related to ocean temperature and circulation anomaly cannot reverse the trend of global warming. The overall trend of climate change is characterized by fluctuating global average air temperature change and constant warming. Such trend cannot be changed by Intermittent and dispersed cold events.

Global warming has seriously undermined natural ecosystems and the economic and social development in China. The areas of crops affected by drought and the fluctuation of grain outputs have increased. The phenological period advances in eastern China and the northern boundary of subtropical and temperate zones moves northward. Ecosystems such as wetland and grassland continue to degrade, and some species have already died out. The frequency of diseases and pests of animals and plants over the country has increased, with significant changes in their distributions. In future, ecosystems will be more vulnerable, threatening species diversity and biodiversity. In addition, the coastal sea level has risen 0.09 m over the past 30 years, slightly higher than global average. Together with frequent typhoons and storm surges, coastal areas suffered more serious impacts. Higher seawater temperatures and surface ocean acidification have led to the degradation of coastal ecosystems. In future, the coastal sea level will continue rise in China. Compared to 2000, the coastal sea level by 2050 will rise between 0.13 m to 0.22 m, thus threatening ecosystems in estuaries and economy in coastal areas. Marine ecosystems including mangroves and coral reefs will continue to degrade. Marine disasters such as storm surge, red tide, salty-tide invasion and salinization will be exacerbated due to sea level rise and extreme climate events. In future, heavy precipitation events in the upper reaches of the Yangtze River are expected to increase, and the triggered geological disasters like landslides and mudslides would have adverse effects on power generation and shipping in the Three Gorges Reservoir. Frost heave in seasonally frozen ground and thaw settlement of permafrost have not only posed a threat to the safe operation of Qinghai-Tibet highway and railway, but also been a prominent problem facing the west-line project of the South-to-North Water Diversion Project in China.

The latest predictions show that compared to the last two decades of the 20th century, annual mean temperature in China is likely to increase by 0.5-0.7°C by 2020 and by 1.2-2.0°C by 2050 and even by 2.2-4.2°C by the end of the 21st century. Temperature warming in North China will be higher than in South China, with the warming in winter and spring higher than the rest of the seasons. By 2020, slight increase of precipitation across the country can be expected. But in North China, the precipitation will be higher than that in South China, while the number of heavy rainfall days will increase in South China. The 21st century is likely to witness more extreme high temperature events in China. In some areas, the number of hot days in summer and the frequency of warm winters and hot summers are expected to increase. More heavy precipitation events will happen. Typhoon and severe convective weather are likely to be more intense, and cold waves in winter will continuously decrease.

Causes of Global Warming

The causes of climate change fall into two categories: natural and anthropogenic forcing. The former includes solar radiation variation, volcanic activity and low frequent oscillation of various components of climate system among others; the latter involves increased GHG concentration in the atmosphere due to fossil fuel use and deforestation, aerosol concentration change, and changes in land use and cover.

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.