For more than 10,000 years, and the entire history of human civilization, the climate has been relatively stable. Now it is not. We’re not moving to a “new normal” but to a world where the climate is constantly changing, meaning that climate risk is also evolving. A stable climate drove how we designed the world around us. The things that humans rely on for survival, including the crops we grow and the infrastructure in cities, are optimized for a set of climate variables that stayed the same over time. That could now put systems at risk around the world.
The overwhelming weight of scientific evidence is that humanity's use of fossil fuels is adding to atmospheric levels of greenhouse gases -- carbon dioxide (CO2) especially -- causing detectable global warming and hence changing the global climate. The evidence shows that CO2 levels have increased by over 30% since the Industrial Revolution and these levels are now at the highest they have been for the past 420,000 years and quite possibly for the past 20 million years.
The Intergovernmental Conference on Climate Change (IPCC) has already established that the average temperature on Earth rose by 0.6°C in the Twentieth Century. It predicts the average temperature will be between 1.4 C and 5.8 C warmer by 2100 than it was in 1990. The sea could rise by between nine and 88 cms. The higher end of these forecasts spells an environmental apocalypse and by the end of the century could mean death for tens of millions. Expected results include a shifting of climatic zones, changes in species composition and the productivity of ecosystems, an increase in extreme weather events (violent storms, flash floods, typhoons, droughts etc) and impacts on human health. People living in coastal areas (which is a large fraction of the population) will be particularly vulnerable. Some diseases will extend their range and natural ecosystems will also suffer.
Anthropogenic change means human-induced change to nature of a natural system. Human activity is having a measurable and negative impact on the climate system. Modification of the surface of the earth induce climatic changes. The main effects are three-fold. First, the ratio of solar radiation reflected from and absorbed by the earth's surface is changed. Secondly, the ratio of convective and evaporative heat released from the earth's surface is changed. Finally, the hydrologic cycle is modified.
With human progress the direct effects of energy and mass transfer upon the atmosphere are acquiring more and more significance. Thermal pollution of the atmosphere is observable over the largest cities. There is the pollution of atmosphere by aerosols and man-made industrial gases, such as soot and oxides of carbon and nitrogen. Humans can also change the climate intentionally: meteorologists are trying to modify the weather, to prevent hail storms or to produce rain.
The Intergovernmental Panel on Climate Change (IPCC) during a coffee break at a workshop. The IPCC was established in 1988 as a response to increasing concern that the observed changes in the Earth’s climate are being largely caused by humans. The IPCC reviews the vast amounts of science being generated around climate change and produces assessment reports every four years.