Geoscience Reference
In-Depth Information
Emission models of the future
A critical problem with trying to predict future climate is predicting the amount of carbon
dioxide emissions that will be produced in the future. This will be influenced by population
growth, economic growth, development, fossil-fuel usage, the rate at which we switch to
alternative energy, the rate of deforestation, and the effectiveness of international agree-
ments to cut emissions. Out of all the systems that we are trying to model into the future,
humanity is by far the most complicated and unpredictable. If you want to understand the
problem of predicting what will happen in the next 100 years, imagine yourself at the be-
ginning of the 20th century and what you would have predicted the world to be like in the
21st century. At the beginning of the 20th century, the British Empire was the dominant
world power due to the industrial revolution and the use of coal. Would you have predicted
the switch to a global economy based on oil after the Second World War? Or the explosion
of car use? Or the general availability of air travel? Even 20 years ago, it would have been
difficult to predict that there would be budget airlines, allowing for cheap flights
throughout Europe and the USA.
The original IPCC reports used simplistic assumption of GHG emissions over the next 100
years. From 2000 onwards the climate models used the more detailed Special Report on
Emission Scenarios. The 2013 IPCC Fifth Assessment report used more sophisticated Rep-
resentative Concentration Pathways (RCPs) which considered a much wider variable input
to the social-economic models including population, land use, energy intensity, energy use,
and regional differentiated development (see
Table 2
). However, the new RCPs mean that
detailed comparison of the 2013 IPCC results is difficult with the IPCC 2001 and 2007 out-
puts, which used the SRES.
There are four main RCPs that are used, defined by the final radiative forcing achieved by
the year 2100, and they range from 2.6 to 8.5 watts per square metre (W/m
2
). Radiative for-
cing is defined as the difference of sunlight (radiant energy) received by the Earth and the
energy radiated back to space. Radiative forcing is quantified at the tropopause, which is
the lowest layers of the Earth's atmosphere where all weather occurs. Its height ranges
from 10 km (~6 miles) at the Poles to nearly 18 km (~11 miles) in the Tropics. Radiative
forcing is measured in units of W/m
2
of the Earth's surface. A positive forcing (more in-
coming energy) warms the system, while negative forcing (more outgoing energy) cools it.
The radiative forcing of Earth can change due to changes in insolation (incident solar radi-
ation) and the concentrations of GHGs and aerosols. The four RCPs were selected to be
