top of page

Backcasting and forecasting 
Understanding the implications of different start dates

By Dr. Lee Anne Willson

Dec. 14, 2021

A first step in getting a Climate Action Plan for Ames was retaining the Pale Blue Dot consultants to do a greenhouse gas inventory. They did this for the year 2018 but with data also to show changes from 2014. The CAP will contain a goal, most likely in the form “A decline of X% from the (starting date) value by (intermediate date) and net zero by (end date).” An obvious starting date is 2018, and the CAP consultant SSG presented several options with this date, a 2030 intermediate date, and a 2050 net zero date. (See the Four Scenarios post)


There has been some discussion of an earlier date than 2018 or 2014, in part because a number of other cities are using 2010 or 2005 and this makes comparison easier. In order to use a date before the Greenhouse Gas Inventory it will be necessary to “back-cast” to figure out what Ames’ emissions were in 2010 or 2005. SSG can use city records and their programs to do the back-casting, should an earlier date be chosen.


Without those resources, we can still look at how some likely values for the earlier dates affect the Climate Action Plan. I’m going to look at three cases: (a) Back-cast from the Business As Planned case, that is, assume that in 2005 and 2010 the total emissions from Ames were about the same as they are now. (b) Use the average per-capita decline in emissions since 2005 to back-cast Ames’ emissions, ignoring population growth (or, assuming that Ames’ above-average efforts were sufficient to cancel population growth). (c) Use the average per-capita decline and population growth to calculate emissions for 2005 and 2010. Looking at these three cases will give us a good feel for the effect of choosing 2005 or 2010 or 2014 instead of 2018 for the starting date of the CAP.


Back-casting from Business as Planned.


The business-as-usual, or more accurately, business-as-planned projection provided by SSG takes into account the actions that are already planned or funding, but does not include things that may or likely will happen in the next ten years. The BAP shows a decline in the next couple of years reflecting ISU’s planned transition from coal to gas, and then a gradual rise with the anticipated growth in the population of Ames (15% by 2040) to reach the same level of emissions around 2050. I’m going to simplify this into: No change in net emissions from Ames from 2005 to 2050. With that simplification, the choice of starting date doesn’t matter, because the emissions are the same for any starting date. The pattern then looks like this:

Screen Shot 2021-12-14 at 11.29.17 AM.png

In the above plot,the emissions follow the red line until 2018 when the deviate along one of the other paths.


Back-casting using average per capita decline for the US (ignoring population growth)

Per capita emissions in the US have steadily declined since 2005, thanks to individual choices and to measures taken by many cities, states, and companies. Fitting the decline in the above plot gives the red line in the plot below.


A city the size of Ames with average per capita declines and no change in population would follow the red line. The three scenarios presented by SSG are shown in the plot for two options: Counting from 2018 (warm colors) or from 2005 (cool colors). For this case, choosing 2005 as a base means that much less needs to be done between now and 2030 – indeed, for the 45% decline from 2005 this projects that we will achieve that just by staying on the red line.


There is another message in this plot: While a decline of 45% or more in our emissions seems like a big step, in fact if we count from 2005 we are on path to achieve that without a big change in what we are doing. Even for the 2018 start dates, the lines are not so far from that red line.


Back-casting using average per capita decline for the US and population growth for Ames


This one essentially assumes that Ames is average in its efforts towards reduced emissions, and Iowa is average in the opportunities it provides. It turns out that the net decline in per-capita emissions 2005-2018 was almost exactly matched by the increase in the population of Ames, so if this is the right description, then our net emissions were flat from 2005 to 2018. That is the same math as the extrapolation of the Business as Planned, and means that whether we take 2005 or 2018 as the zero date makes no difference in what needs to be achieved by 2030.




Choosing an earlier initial date matters only if the total emissions were higher (or lower) at the date chosen than they were in 2018. Using the emissions per capita for the US and including the growth in the population of Ames since 2005 would give no net reduction. In reality, Ames has been pro-active and so there has been a net decline in several areas – in electric power production with the shift from coal to gas, the addition of wind energy, and the solar farm.


In either case, Figure B shows that the scenarios being presented by SSG are not so very far from what Ames would be doing anyway, assuming it continues its forward-looking efforts towards sustainability.


Having a goal


So why do we need a CAP, then? Well, think about a time when you decided to adopt a budget, save for something special, go on a diet, adopt an exercise program, or accomplish some other improvement in your life that requires ongoing effort and attention. If you did this by simply saying “OK, I’m going to save more/eat less/exercise more” then probably not a lot changed. But if you sat down and worked out that budget, and found that in order to make your goal you would need to give up those expensive lattes, or cancel some of your streaming subscriptions, or set aside an hour a day for exercise, then you are more likely to achieve your result. In the same way, having a CAP with a goal means looking harder at all the things that may add up to the desired outcome.


In the presentation to the SIC in December, SSG showed how the detailed plans for two cities compared, one of them for a city that adopted a goal and one for a city that did not.


Nearly everyone at the session was able to pick out which was which: The city with a goal included many more actions they would be taking than the city without a goal. Only with a goal will there be an incentive to look at every opportunity to make a difference, even if some of those individually make a small difference. It is the nature of the problem that we will need a hundred 1% solutions, not a few big ones, to keep the climate where it supports life as we know it.


Just as with your budget, or your diet, you may do your very best to follow the plan but it may be derailed by events. Your car may need repairs, or you end up in a situation where you can’t avoid eating more than you should, or an expected source of income dries up, so it may also be true that the best plans for reducing emissions may run into a snag. It is important to think of the goal in the Climate Action Plan as an indication of what we hope to be able to do, and not as a contract where there are penalties for failure. Indeed, unexpected developments may slow progress, or they may speed it up. An essential feature of the plan is a regular review and, if needed, adjustments of the dates or % reductions.

bottom of page