While climate modeling is complicated, the evidence is increasingly strong. The environmental impact of burning fossil fuels for energy at a global scale is significant. Since the beginning of the 20th century, sea levels have risen 7-8 inches and Earth’s surface temperature has risen by 1.8°F globally. Any warming may hurt us—rising sea levels, more forest fires, and more extreme weather, along with changes in everything from precipitation patterns to ocean currents. But non-linear dynamics and unpredictable second-order effects may make reversing the damage harder the further we go.
In a complex system, it's very hard to know how far is too far. But the precautionary principle makes drastic changes worrisome. The United Nations Framework Convention on Climate Change deemed 2°C (3.6°F) above pre-industrial levels to be the maximum tenable global temperature increase. Beyond that, the risk of irreversible harm looks unacceptable.
The Intergovernmental Panel on Climate Change (IPCC) estimates that we cannot cumulatively emit more than one trillion metric tonnes of carbon while remaining below 2°C of warming. Earth is already halfway to that emission threshold and may exceed it by 2050.
Emission scenarios predict the extent and severity of humans’ potential impact.
The IPCC explores potential scenarios of human activity on future rates and magnitudes of climate change through Representative Concentration Pathways (RCP). RCPs take into account both climate dynamics and real-world factors like population growth, environmental policy, and development of new technologies. There are three commonly-used RCP scenarios: 8.5 (high), 4.5 (medium), 2.6 (low). To date, our global emissions trajectory and climate policy outlook most closely resemble RCP 8.5.
The charts below show how each RCP scenario breaks down across three key indexes of climate change: composition of renewable energy production, global surface temperature, and atmospheric CO2 emissions and concentration. As demonstrated in the graphs, RCP 2.6 provides the best chance of reducing the danger threshold beyond the risk of irreversible harm to the environment.
RCP 2.6Low emissions
We can reasonably expect that this RCP will limit the increase of global mean temperature to 2 degrees Celsius by reducing the emissions of greenhouse gases from 2010 to 2100 by nearly 90%. This outcome will require immediate progress in energy efficiency, increased deployment of renewable and nuclear power, and widespread use of carbon capture, storage, and bioenergy.
RCP 4.5Medium emissions
Assumes a worldwide effort to reduce emissions by roughly 50% from 2010 to 2100 via policy change and technological developments, leading to CO2 emissions peaking mid-century and declining thereafter. This scenario could keep us under the required two degrees of global warming, but would require half of the world's global energy production to come from renewable sources by 2060.
RCP 8.5High emissions
Represents a likely 'business as usual' outcome if there are no policy changes to reduce emissions, and only modest technological improvements and behavioral changes. Greenhouse gas emissions will rise by a factor of three over the course of the century due to a high fossil-intensity and high demand of the energy sector.