[dropcap letter="L"]
Most climate scientists agree that the main cause of the current global warming is the human extension of the greenhouse effect —the warming that results when the atmosphere traps heat radiated from Earth into space—.
Some gases in the atmosphere prevent that heat from escaping. Long-lived gases that partly remain in the atmosphere and do not respond physically or chemically to temperature changes are called "drivers" of climate change. Gases like water vapor that do respond physically or chemically to temperature changes are called "feedbacks." Thus, the gases that contribute to the greenhouse effect are:
- Water vapor: it is the most abundant greenhouse gas but, above all, it acts in feedback with the climate. The higher the temperature of the atmosphere, the more water vapor and the greater the probability of clouds and precipitation. These are the most important feedback consequences of the greenhouse effect.
- Carbon dioxide (CO2): it is a minor but very important component of the atmosphere. It is released in natural processes such as respiration and volcanic eruptions, as well as through human activities such as deforestation, land-use change, and the burning of fossil fuels. Since the beginning of the Industrial Revolution, the concentration of CO2 has increased by a third. It is the most important long-lived "driver" gas of climate change.
- Methane: it is a hydrocarbon gas of natural origin and a result of human activities such as the decomposition of landfills, agriculture (especially rice cultivation), the digestion of ruminants, and the management of livestock and production animal waste. On a molecular scale, methane is a much more active greenhouse gas than carbon dioxide, although less abundant in the atmosphere.
- Nitrous oxide: it is a very powerful greenhouse gas produced mainly in cultivation through the use of commercial and organic fertilizers, the incineration of fossil fuels, the production of nitric acid, and the burning of biomass.
- Chlorofluorocarbons (CFCs): these are synthetic compounds of industrial origin that were used in various applications, now widely regulated in their production and release into the atmosphere to prevent the destruction of the ozone layer.
- Overall, the Earth will become warmer. Some regions may welcome warmer temperatures, but others may not.
- A warmer climate will likely lead to more water evaporation and precipitation, but the impact by region will be variable: some will become wetter, others drier.
- A stronger greenhouse effect will warm the oceans and melt some ice and glaciers, which will increase sea level. Ocean water will expand as it warms, causing even more sea level rise.
There is a greater than 90% probability that, over the last 250 years, human activities have warmed our planet.The role of human activity
In the Fifth Assessment Report, the Intergovernmental Panel on Climate Change (IPCC), a team of 1,300 independent scientific experts from countries around the world and under the auspices of the United Nations, concluded that there is a more than 90% probability that, during the last 250 years, human activities have warmed our planet. In the last 150 years, the industrial activities on which our modern civilization is based have increased carbon dioxide levels in the atmosphere from 280 to 400 parts per million. The expert group also concluded that there is a greater than 90% probability that human-emitted greenhouse gases such as carbon dioxide, methane, and nitrous oxide have caused most of the increase in Earth's temperatures over the last 50 years. According to these experts, it is very likely that the rate of increase in global warming due to these gases is an unprecedented event in the last 10,000 years or more.
Solar Radiation
Given that the sun is the main source of energy that drives our climate system, it is reasonable to assume that changes in the sun's energy output are the cause of changes in the climate.
In fact, several studies have shown that solar variability has played a key role in past climate changes. For example, a decrease in solar activity is believed to have triggered the Little Ice Age (LIA) from 1650 to 1850, as well as the isolation of Greenland and the advance of glaciers towards the Alps between the years 1410 and 1720.
However, several lines of evidence show that the current climate change is not due to changes originating from solar energy:
- Since 1750, the average amount of energy from the sun has remained constant or increased slightly.
- If the temperature were caused by increased solar activity, scientists would expect to see hotter temperatures in all layers of the atmosphere. However, they have observed a cooling in the upper atmosphere, and a warming at the surface and in lower parts of this layer. This is because greenhouse gases are trapping heat in the lower atmosphere.
- Climate models that include changes in solar radiation cannot reproduce the observed temperature trend over the last decade without an increase in greenhouse gases.
Global climate change already has visible effects on the environment. Glaciers have shrunk, ice on rivers and lakes is melting ahead of time, plant and animal habitats are changing, and trees are blooming earlier. Thus, the consequences of global climate change that scientists predicted in the past are already taking place: melting of the seas, acceleration of sea level rise, and more intense and longer-lasting heat waves.
Scientists are certain that the global temperature will continue to rise in the coming decades, largely due to greenhouse gases resulting from human activity. The Intergovernmental Panel on Climate Change (IPCC) predicts a temperature increase of between 1.4 and 5.5 degrees Celsius in the next century.
According to the IPCC, the extent of the effects of climate change in regions will vary over time, as will the adaptive capacity of different environmental and social systems. Furthermore, the IPCC predicts that three degrees Celsius above 1990 levels will produce beneficial impacts in some regions and detrimental ones in others. As the global temperature increases, annual net costs will increase over time. "In total, the range of published evidence indicates that the net cost of damages from climate change is significant and will increase over time," explains the Intergovernmental Panel on Climate Change (IPCC). According to its Third Assessment Report, some of the long-term effects of climate change will be the following:
Change will continue throughout this century and beyond. Beyond the next few decades, the magnitude of climate change will fundamentally depend on the amount of greenhouse gases emitted globally, and how sensitive the Earth's climate is to these emissions.
Temperatures will continue to rise. If we add the warming resulting from human activity to a naturally variable climate, the temperature increase has not been, and will not be, uniform and even across the planet.
Frost-free growing seasons will lengthen. The length of frost-free periods (and corresponding growing seasons) has increased since 1980. In a future where greenhouse gas emissions continue to rise, an increase of a month or more in frost-free growing periods will occur by the end of the century.
Change in precipitation patterns. Average precipitation has increased since 1900. Throughout the century, wetter winters and more spring rains are expected. Climate projections suggest that the current trend of increasing and heavy precipitation will continue to rise.
More droughts and heatwaves. More frequent and intense droughts and heatwaves are also expected. Additionally, summer temperatures will continue to rise and soil moisture will decrease, further exacerbating heatwaves. By the end of the century, extreme heat days previously recorded every two decades will occur every two or three years in areas like the US, for example.
Stronger and more intense hurricanes. The intensity, frequency, and duration of North Atlantic hurricanes, as well as the frequency of the strongest ones (categories 4 and 5), have increased since the early 1980s. The imprint of human activity and natural causes remains uncertain in this increase. However, as the climate warms, more hurricanes with intense rains and storms are anticipated.
The Arctic will melt. It is expected that, by mid-century, the Arctic Ocean will be practically ice-free during its summer season.
Sea level will rise by 2100. Since reliable measurements began in 1880, sea level has risen by about 20 centimeters. It is estimated to rise between 30 centimeters and 1.2 meters by 2100. This phenomenon is due to the excess water resulting from melting ice and the expansion of the sea as it warms.
In future decades, sudden storms and high tides will combine with rising sea levels and flooding in many regions of the planet. The rise in sea level will not stop in 2100, as oceans need a long time to respond to a warmer Earth's surface. Therefore, for several more centuries, ocean waters will continue to warm and sea levels will rise to levels equal to or higher than those that occurred in this century.