Chapter 3

Chapter 3: FUTURE CLIMATE SCENARIOS

Over the 4.5 billion year history of Earth, the climate has fluctuated between extremes
of severe hothouse conditions with average surface temperatures up to 28°C and a
snowball earth where snow and ice covered the entire surface. These transitions have
reshaped oceans and land masses, eradicated habitats and caused the extinction of
species while facilitating the emergence and proliferation of new species.

The age of mammals began 66 million years ago under hothouse Earth conditions driven
by atmospheric CO2 concentrations in excess of 2,000 ppm. Declining greenhouse gas
levels following the Arctic Azolla event 49 million years ago led to a gradual cooling
of the planet with a transition to the current ice age beginning 2.6 million years ago.
The 14°C difference between current temperatures and the hothouse earth of the
past is used by some as the basis for arguments that the 1°C increase in temperature
since the onset of the Anthropocene is of no consequence, and that the planet can
readily withstand the relatively modest impacts of a man‐made imbalance in the global
carbon cycle.

Earth is in no danger of demise based on the activities of man. However, continued
excess rates of anthropogenic GHG emissions places the civilizations built by man,
along with existing habitats for other lifeforms at risk.

Arguments that the planet has naturally passed through extreme climate variation
as the basis for a continuation of current practices for fueling the Anthropocene are
absurd and irresponsible to future generations. Modern cities, industries, and food
supply chains were built during the briefest of moments in the history of Earth. This
moment occurred during an interglacial period within an ice age characterized by 280
ppm atmospheric CO2 and associated stable average surface temperatures, weather
patterns, ice masses, sea levels and ocean pH. There is some degree of tolerance
for anthropogenic changes to these baseline climate conditions; however, we are
approaching the limit beyond which climate change will negatively impact important
sectors of global civilization and will damage the habitats and viability of other species.