The Earth’s climate system has changed over many millions of years. The information gathered from natural archives offers a long-term perspective on observed changes and expected changes over the centuries that follow. Additionally, these past climate reconstructions demonstrate a strong relationship between atmospheric CO2 concentrations and global surface temperatures. Depending on the emissions pathway that is followed, levels of global warming that have not been witnessed in millions of years could be reached by 2300.
The current climate globally is already different from the climate of the early or mid-20th century with respect to several climatic impact drivers (CIDs), resulting in shifting magnitude, frequency, duration, seasonality, and spatial extent of associated climate indices.
Heat & Cold: Changes in temperature-related CIDs, such as average temperatures, the length of the growth season, and the frequency of intense heat and cold, have already occurred.
Wet and dry: Increased greenhouse gas emissions have driven increased contrasts in precipitation amounts between wet and dry seasons and weather regimes over tropical land areas. Most land areas have experienced decreased available water in dry seasons due to human-induced climate change associated with changes in evapotranspiration.
Wind: The global proportion of major tropical cyclone (TC) intensities over the past four decades has increased. The proportion of intense TCs, average peak TC wind speeds, and peak wind speeds of the most intense TCs have increased globally with increasing global warming.
Causation
The principal natural catalyst of climate change include changes in incoming solar radiation, volcanic activity, orbital cycles, and changes in global biogeochemical cycles. In comparison, anthropogenic climate change is due to CO2 emitted by the combustion of fossil fuels (petroleum, coal, natural gas). Changes across a more significant number of climate system components, including changes in regional climate and extremes, can now be attributed to human influence.
Human influence is the main contributor to the observed increase in the intensity and frequency of hot extremes and the observed decrease in the intensity and frequency of cold extremes on continental scales. Also, it has contributed to the intensification of heavy precipitation in continents.
Progress
Earth system observations are an essential driver of progress in the understanding of climate change. Improved measurements of ocean heat content, warming of the land surface, ice-sheet mass loss, and sea level changes have enabled more robust reconstructions of the spatial and temporal patterns of past climate changes over multiple time scales.
Understanding Climate Variability and Emerging Changes: Climate change is clearly evident globally and is becoming more perceptible at the regional and local spatial dimensions.
Carbon Cycle
The continued growth of atmospheric CO2 concentrations over the industrial era is unequivocally due to emissions from human activities. Ocean and land carbon sinks slow the rise of CO2 in the atmosphere. Carbon sinks for anthropogenic CO2 combine with the main physical ocean and biospheric land processes that drive carbon exchange between multiple land, ocean, and atmospheric reservoirs.
Water Cycle
Human-caused climate change has driven detectable changes in the global water cycle since the mid-20th century. It will cause substantial further changes at both global and regional scales. The severity of very wet and very dry events increases in warming, but changes in atmospheric circulation patterns affect where and how often these extremes occur. Water cycle variability and related extremes are projected to increase faster than mean changes in most regions of the world.
Land-use change and water extraction for irrigation have influenced local and regional responses in the water cycle. Large-scale deforestation likely decreases evapotranspiration and precipitation and increases runoff over the deforested regions relative to the regional effects of climate change.
Climate Future
Depending on greenhouse gas emissions, the amount of global warming this produce, and how the climate system reacts to that warming, people will experience climate change this century and into the next.
Image: Different social and economic developments can lead to substantially different future emissions of carbon dioxide (CO2 ), other greenhouse gases, and air pollutants for the rest of the century.
Conclusions
Limiting further climate change would require substantial and sustained reductions of GHG emissions. Without net zero CO2 emissions and a decrease in the net non-CO2 forcing, the climate system will continue to warm. Deliberate carbon dioxide removal (CDR) from the atmosphere has the potential to balance current CO2 emissions to reach net zero CO2 emissions or to generate net negative CO2 emissions. This means enhancing biological or geochemical carbon sinks or by direct capture of CO2 from the air.
Future emissions will determine the environment we and the next generation will experience. Rapid emissions reduction will prevent more changes, while sustained emissions will bring about bigger, faster changes that will increasingly affect all regions. Some changes will last for hundreds or thousands of years, so the decisions we make today will have long-term effects.
The article is based on the report: The Climate Change 2021
