The recent Inter-Governmental Panel on Climate Change Special Report rather alarmingly gave us all 12 years to save the world. But the temperature rises being caused by greenhouse gas emissions seem small, and the differences between the various scenarios even smaller. What difference would a 2.0°C rise make, instead of 1.5°C? Not much, surely. The IPCC report tells us why the difference is significant.
Climate change represents an urgent and potentially irreversible threat to human societies and other life on the planet. In recognition of this, the overwhelming majority of countries around the world adopted the Paris Agreement in December 2015, the central aim of which was to limit global temperature rise to 1.5°C above pre-industrial levels. Human-induced warming has already reached an increase of 1.0°C at the time of writing of the Special Report. If the current warming rate continues, we would reach human-induced global warming of 1.5°C around 2040, and countries’ pledges to reduce their emissions are currently only in line with limiting global warming to 3.0°C. It's also worth noting that we're talking about an average increase - in many places it will be higher.
Reaching 2.0°C instead of 1.5°C of global warming would lead to substantially warmer extreme hot days in all land regions. It would also lead to an increase in heavy rainfall events in some regions, particularly in the high latitudes of the northern hemisphere, potentially raising the risk of flooding. In addition, some regions, such as the Mediterranean, are projected to become drier at 2.0°C versus 1.5°C of global warming. The impacts of any additional warming would also include stronger melting of ice sheets and glaciers, as well as increased sea level rise, which would continue long after the stabilization of atmospheric CO2 concentrations.
Change in climate averages and extremes will have knock-on effects for the societies and ecosystems living on the planet. Climate change is projected to make the poor poorer and increase the total number of people living in poverty. The 0.5°C rise in global temperatures that we have experienced in the past 50 years has contributed to shifts in the distribution of plant and animal species, decreases in crop yields and more frequent wildfires. Similar changes can be expected with further rises in global temperature.
The lower the rise in global temperature above pre-industrial levels, the lower the risks to human societies and natural ecosystems. Put another way, limiting warming to 1.5°C can be understood in terms of ‘avoided impacts’ compared to higher levels of warming. For example, thermal expansion of the oceans, resulting from delayed ocean mixing, means sea level will continue to rise even if global temperature is limited to 1.5°C, but this rise would be lower than in a 2.0°C warmer world. Ocean acidification, the process by which CO2 dissolves into oceans and makes them more acidic, is expected to be less damaging in a world where warming is stabilized at 1.5°C compared to 2.0°C. The persistence of coral reefs is greater in a 1.5°C world than that of a 2.0°C world, too. In another example the report tells us that that the number of species projected to lose over half of their climatically determined habitat is doubled for plants and vertebrates and tripled for insects at 2.0°C versus 1.5°C of warming.
Limiting the increase to 1.5°C would also make adaptation easier. Adaptation is the process of adjusting to changes in climate and its effects. Even though climate change is a global problem, its impacts are experienced differently across the world. This means that responses are often specific to the local context, and people in different regions are adapting in different ways. A rise in global temperature from the current 1.0°C above pre-industrial levels to 1.5°C, and beyond, increases the need for adaptation. Therefore, stabilizing global temperatures at 1.5°C above pre-industrial levels would require a smaller adaptation effort than at 2.0°C.