Final answer:
To limit global warming to +2 degrees Celsius, atmospheric carbon levels must be kept well below 560 ppm, as doubling pre-industrial CO2 levels to 560 ppm is projected to lead to a temperature rise of about 3.0°C. The IPCC suggests that levels reaching 450 ppm are already dangerous, highlighting the need for lower CO2 concentrations to ensure safety.
Step-by-step explanation:
The question relates to determining the atmospheric carbon levels required to limit global warming to a +2 degrees Celsius increase. Scientists have used various models and calculations to provide insights into this relationship between atmospheric carbon dioxide concentration and temperature rise. Pre-industrial levels of CO2 were 280 ppm, and it has been suggested that the Earth has already experienced an approximately 1.0°C increase with the likelihood of reaching 1.7°C based on current levels. To keep the temperature rise to around 2°C, it is necessary to consider both the direct effect of increased CO2 levels and the feedback mechanisms that can amplify this temperature response. Based on the climate sensitivity parameter, a doubling of the pre-industrial CO2 levels to 560 ppm would lead to a radiative forcing of about 3.7 W/m² and, with feedbacks, a temperature increase of about 3.0°C, far exceeding the 2°C threshold. Considering these factors, staying well below 560 ppm would be critical. Additionally, the IPCC has suggested that 450 ppm of CO2 is already dangerous, indicating that even lower levels might be required to safely limit global warming to +2 degrees Celsius. Restoration of ecosystems and conservation strategies are also presented as crucial adaptations needed to mitigate the impacts of current and future CO2 levels.