How Soon? How fast?

How soon do we need to get started reducing greenhouse emissions?   And when we do start, what speed do we need to switch over to alternative fuels?

To avoid despair at seeming impossibility or complacent waiting for the right time to act, a close look at the parameters is required.

Before you answer the "how soon" question with "Twenty years ago!", the good news is that we already have.  We have long had hydro and nuclear power stations delivering carbon free electricity. Had we not, emissions and atmospheric concentrations would be higher today.  More good news is that non-hydro renewable energy in the U.S. is now greater than hydro.

 Fig. 1.  Renewable Energy Electricity Generation

To answer the "how fast" question, we look at the Paris Agreement global targets in annual billion metric tons of combined greenhouse gas emissions in CO2-equivalents (CO2e).  Charted below, we find both the time period and the slope:

Fig. 2.    Global Annual Emissions (gigatons/yr)

By 2050, over a span of thirty years, emissions are reduced to a level where most of the temperature increase has already happened.  (Temperature rise roughly follows total emissions, the area under each line.)   Thirty years for one technology to take over another is quite common.  Automobiles replaced horses in 30 years.  Cell phone are replacing land lines at that rate.

Figure 2 assumes an immediate level of replacement energy, added every year.  Reaching that level won't happen overnight.  Factories need to be built, workers trained, and power sources installed. 

How does the U.S. achieve this?  Fossil fuels make up about 80% of U.S. energy consumption.  To make the 2 degree goal we need 10-fold increase over our existing rate of new renewable energy.  For the 1.5 degree goal, the factor is 15.  If we delay 15 years, the required increase becomes 20x.

Is this level of industrial increase doable?  A look back at a couple examples tells us, yes, easily.  One is the combat aircraft production during World War II compared to production in 1941. Another is Ford Model T after its first year of production.

Fig 3.    Increase in Production

  (Note how the war's rapid increase in production rate contrasts to Ford's 14 year build-up.)

The aircraft production grew from 1,771 a year to 74,564 per year, a 42x increase.  Ford increased production from 10,666 to 170211, a 16x increase.  Then Ford went on to make as many as 2 million Model T cars a year, 187 times the first year production.

We don't need rates of that sort, unless we wait too long. We can achieve ellimination f fossil fuels by 2050 by implementing a 30% growth in our current growth of carbon-free energy each year for the next ten years, then maintain the 2030 replacement rate for the next 20 years.  This 30% increase is far below Ford's 10 year average of 55%/year.

 Fig 4.   Total Global Emissions - projected

Figure 4 shows the accumulation of annual carbon emissions for the next 40 years.  Estimates center around 1500 gigatons CO2e for a 2 degree rise.  A 1.5 degree target calls for no more than 1000 gigatons.  Lines A and B represent immediate and steady replacement of fossil fuels as shown in Figure 2.

If we keep on with business as usual, we will have reached the 1.5 degree level by 2036, and the 2 degree level around 2043.  These are estimates, but they point to a date of 2030 by which significant progress must be made. 

What this means is the window of opportunity is small.  Ideally, we ramp up renewable energy production to a reasonably high level by 2030, replacing fossil fuel's 80% share of energy in the following twenty years.

Waiting too long, where we need a three or four year crisis mode push starting in 2037 is not a desirable choice.

Bottom line:  The Paris Agreement targets demand a 30 year time frame, a ten year ramp up in production is achievable, and historically both are quite normal.