The Anthropocene -
we have to become sustainable
residents of our only planet

Michael P.C. Watts
"It’s the sea level stupid" probably should be the motto for our current planetary concerns, not global warming. In the geological past,  small
changes in temperature of a few degrees have caused sea levels to be 120m lower and 70m higher than it is  today.  Changes to the equilibrium
energy balance of the planet happen over thousands of years, so what appears to be an easily manageable rise of 0.5-1 m in the next 100 years
will become 10’s of meters if we do nothing to return the planet to its pre-industrial equilibrium. It is difficult to overstate the cost of the disruption
from 10’s  of meters sea level change.

With the global warming debate raging in spite of the consensus in the climate science community, this blog and the accompanying paper is my lay-
persons attempt to distill the discussion into a few simple ideas that seem to me to  illustrate of some the key questions; is the globe warming, what
is causing it , what is going to happen, how quickly is it going to change, what are our options ?.

There is no question that the globe is warming, even though it is challenging to reliably measure the average temperature of a planet with daily, and
seasonal variation, large oceans and patchy data collection locations.  The sceptics complaints about the historical record have been debunked by
the work of a noted climate sceptic, Muller of UC Berkeley who was hired by the Koch Brothers to independently evaluate the data. Muller concluded
that there has been a 0.9C  increase in temperature over the last 50 years, almost identical to the estimate by NOAA. Our globe is waming.

Insight into the causes of the temperature changes comes from multiple independent sources; theory, our local planetary neighbors, the ice age
record and data from last 100 years .

A theory for the factors that control planet wide  temperature was first described by  Svante  Arrhenius in 1896. He showed that because carbon
dioxide was transparent at visual wavelengths and absorbs in the infra – red, it was the key to the earth being a comfortable place to live rather
than 15C colder.   

The second source is the understanding that  the surface temperature of Venus (450C) is the result of an atmosphere of almost pure CO2, and is
an example of “runaway greenhouse effect”.

The third source comes  from the Vostok ice cores that showed that temperature has tracked CO2 levels for over 400,000 years through 4 ice ages.

The fourth source is the last 100 years of data from the earth that shows that CO2 levels have increased 40% above pre-industrial levels and are
caused by our use of fossil fuels. The increasing CO2 levels correlate with the temperature increases confirmed by Muller, and match the prediction
of Arrhenius over 100 years ago.

The sceptics correctly point out that these historical data confirm correlation, not cause and effect. The trouble is that proving cause and effect is
challenging. For an example, consider what it takes to prove that a drug is effective; multiple subjects that  experience different levels of a drug or a
placebo with a double blind so there is no chance of inadvertent bias.   We only have one earth like planet to experiment on, so it is impossible to
prove cause and effect to a statistical certainty. However multiple data sources are consistent with theory so, while it is possible that higher CO2
levels from fossil fuel use MIGHT not affect climate, the probability has to be very small. The cause of warming is almost certainly rising CO2 levels
from fossil fuel use.

The ice ages give a clear view of the long term planetary impact of temperature changes. The ice age cycles in the last 400,000 years  produced a
-5 to -8C change in temperature and a 120 meter sea level drop, shown in Figure 1a. Geological data from 3-5M years ago (Pliocene), shows that
temperatures were 3-5C higher and sea levels 20-30 m higher. In the time of the dinosaurs 100M years ago, there were no ice caps and sea levels
were 70 m higher. The critical point is that takes 1000’s of years for the sea level to change in response to changes in temperature, although
changes can be more abrupt when individual land based ice caps collapses. I have illustrated potential future sea level scenarios also in Figure 1a.
Fig 1 Sea level history and an illustration of  future prospects. a) Data from the last ice age (1) superimposed on an illustration of future
scenarios with warming producing sea levels of past Pliocene or Jurassic periods after rising at a rate of 1 m per 100 year. b) An immediate or
delayed freeze on CO2 emissions. c) Map of coastline for different sea level scenarios.

The result of sea level changes on coastline are shown in  Figure 1c above. In a detailed review of 259 climate references, Jim Hansen of NASA
fame (2)  considers that;

“The carbon from fossil fuel burning will remain in and affect the climate system for many millennia, ensuring that over time sea level rise of many
meters will occur”

The record shows that unconstrained CO2 emissions, and other greenhouse gases such as methane, guarantee tens of meters of sea level
increase as seen in the geological record.

Hansen describes the real question as:

"The important point is that the uncertainty is not about whether continued rapid CO2 emissions would cause large sea level rise, submerging
global coastlines – it is about how soon the large changes would begin."

In the next 100 years, 1-4C rise is predicted with 0.5-1 m of sea level rise. Even  if  CO2 levels stayed constant and temperatures remain at
today's levels,  historical data show that sea levels will continue to rise for over 1000  years and produce many meters of sea level.   
The uncertainty as to timing centers on the stability of the land based ice caps  that will determine the timing of sea level rise, The news of
melting in Greenland, and the break of the ice pack in Antarctica are alarming. The quicker the change, the more difficult it will be for all the life
on our planet to adapt.  

Some action scenarios were considered by Hansen;

"If CO2 emissions were frozen today, CO2 levels and temperatures would return to average levels for the recent past by 2100. A 40 year delay
before freezing, would delay return to average levels for 1000 years and create meters of sea level change."

I have illustrated these  sea level scenarios in Fig 1b.

The impact of 10’s of meters of sea levels change can be appreciated in the map in Fig 1c above. Coastal cities will have to be relocated, and
huge populations in low lying countries such as Bangladesh will have to be relocated as well. The costs would be unimaginable.

The question of what to we need to do is really self evident. If we do not become independent from fossil fuels now, we will have to eventually as
they are a finite resource. As a result, in reality there is little financial  penalty from eliminating fossil fuels.  If we do nothing, our distant offspring
will have to deal with massive planet wide disruption, AND becoming independent on fossil fuels.

Humans moved from living off the land as nomad’s to modifying the planet with domestic agriculture around 9,000 years ago. Modern civilization
evolved while the sea level was stable and stable fixed communities could develop. If we want to last another 9,000 years, there really is no
choice other than becoming sustainable residents, rather than consumers, of our planet. - SOON.

1) Replotted from Fig 1A of Lambeck
2) Hansen,

Further references and additional discussion can be found in the updated accompanying paper.

 Copyright M.P.C.Watts 2017
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