1.5: Summary- What Have We Learned So Far?

• Many scientists propose that the current era be called the Anthropocene in view of the fact that human beings have emerged as a major force transforming the planet, comparable to major geological events.
• Human impacts on climate began gradually with the Agricultural Revolution that started 10,000 years ago; the rate of transformation picked up with the Industrial Revolution that started around 1750. Two hundred years later, there was a quantum jump in the pace of transformation with the Great Acceleration beginning in 1950.
• The post-1950 period witnessed massive changes in the composition of the atmosphere due primarily to the use of coal and petroleum for power generation, transportation, and industries.
• Climate change caused by emissions of greenhouse gases and black carbon has emerged as one of the iconic impacts of the Anthropocene.
• The primary sources for anthropogenic CO2 emissions are fossil fuel combustion, biomass burning, cement manufacturing, and deforestation and other land use changes. Methane sources include natural gas leaks during production, processing, and transmission; wood burning; cattle and other livestock; rice paddy agriculture; and landfills and sewage water treatment plants. Sources for CFCs and HFCs are refrigeration and air conditioning. Ozone is not directly emitted by human activities, but the emissions of ozone precursor gases (methane, nitrogen oxides, volatile organics) produce ozone in the lower atmosphere. Nitrous oxide is released by agriculture as a result of fertilization. Black carbon is produced by diesel combustion; burning of solid coal; and burning of firewood, crop residues, and dung for cooking.
• The emitted greenhouse gases cover the planet like a blanket and trap the heat energy (infrared radiation) emitted by the surface. This trapped heat energy warms the planet. Carbon dioxide is the most important warming pollutant, contributing as much as 55% of the present-day warming. Other greenhouse gases and black carbon particles contribute the remaining 45%. The planet has already warmed by about 1°C because of this added heat. All parts of the Earth system, including the atmosphere, land, oceans, glaciers, and sea ice, are warming. The warming has extended down to 700 meters below the ocean surface and up to about 12 kilometers in the atmosphere—just as predicted by climate science and climate models. The last time the planet was this warm was during the Eemian interglacial period of 130,000 to 115,000 years ago. Impacts of this warming include heat waves, severe storms, droughts, and sea level rise.
• Climate change science is intensely data driven. The changes in the planetary climate have been documented by thousands of instruments at the surface and aboard ships, aircraft, balloons, and satellites. These data have been integrated into sophisticated climate models run by the world’s fastest computers to determine the causes and impacts of climate change.
• The validity and veracity of models have been assessed by simulating climate changes during the twentieth century and so far in the ­twenty-first century, and then comparing the models’ predictions against observations. Predictions that have been verified include when human-induced warming would be detected above the background natural variations; amplifying feedbacks involving water vapor, sea ice, and sea level rise; and the depth of penetration of the warming in the oceans and the atmosphere.
• The observed 1°C warming has already led to a substantial retreat of sea ice, an increase in hurricane intensity, an increase in the intensity of precipitation worldwide, large-scale droughts and more frequent fires, and a tenfold increase in extreme temperatures and lethal heat waves. Major health impacts have also been documented.
• The planet is currently on a path to warm to 1.5°C (from preindustrial levels) within 15 years, most likely by the year 2030. If we keep adding climate pollutants at the present rate, it will continue to warm to 2°C by 2050, and to a catastrophic 3°C to 7°C (95% confidence range) by the end of this century. The potential impacts on human health, ecosystem health, and species extinction lead to the conclusion that warming in excess of 5°C would pose existential threats to Homo sapiens (all 11 billion of us) and numerous other species.

As we have seen in this chapter, the science of global warming is clear, and the potential impacts of continuing emissions for human society and natural ecosystems are severe. There is still time to act, but we have only about 20 years to bring all of the solutions described in this book up to full speed.

Fortunately, a range of climate solutions that offer real hope are available and will be explored in detail in the remainder of this book. These solutions will help provide you, climate warriors, with the tools to avoid such a catastrophic future.