Why We Need to Understand 'Short-Term Carbon Emissions'

In a recent post, "Why the World Needs Carbon Literacy," I wrote that I was most concerned about upfront carbon emissions that happen in the product and construction process stages, writing that "I have a short attention span and am not really interested in the end-of-life emissions; I worry about the now."

In a recent article in ICIBSE Journal, Nigel Banks of Ilke Homes, a United Kingdom modular builder also worries about the now and writes that we need to focus on "short-term" emissions. This is an important addition to carbon literacy. Banks writes:

"What is clear from COP26 is that we all need to deliver significant emissions reductions this decade. As designers, this means understanding better the emissions resulting from our designs and, potentially, challenging some of our preconceived ideas of what delivers low carbon or zero carbon buildings."

We have noted many times that every ounce of carbon dioxide (CO2) emissions adds to global warming and brings us closer to the carbon budget/ceiling that we have to stay under to keep global heating under 2.7 degrees Fahrenheit (1.5 degrees Celsius), which is why we have to cut emissions almost in half by 2030, and why the timing of the emissions matters.

What Banks has done that is so interesting is look at the upfront carbon emissions and a defined period of operating emissions together, calling that "short term" emissions. Since upfront emissions vary with the amount of stuff that you put into a building, he is trying to find the sweet spot where you might dial down the upfront carbon and dial up the operating carbon to find the lowest overall short-term emissions, the emissions that really matter if we are going to stay under that carbon ceiling.

Banks is Special Projects Director for a modular housing company that has developed a line of zero-carbon homes, so he has a real economic interest in finding that sweet spot. Homebuyers care a lot more about upfront costs than they do about upfront carbon.

Banks' math only works when one electrifies everything and the electricity is low carbon—otherwise operating carbon emissions dominate the picture really fast. Banks gives two examples: one on the left, where he compares double and triple glazing a window, and on the right, where he compares 120mm (4.7") of mineral wool insulation to 180mm (7"). The black horizontal line is the added upfront carbon, the red line is the increased operating emissions with a gas furnace, and the green line is the increased operating emissions with clean electricity and an air source heat pump. It's clear in both scenarios that from a short-term carbon point of view, one is better off not adding the insulation or the extra pane of glass.

Banks tells Treehugger he is being "deliberately provocative" with his statements here. But this will give comfort to the "fist pumps for heat pumps" and electrify everything gang in the U.S., who think efficiency is no longer something to worry about in an all-electric world.

Banks writes:

"Hopefully, everyone is aware that the electricity Grid has significantly decarbonised and that a grid-connected heat pump delivers very low – and, increasingly, close to zero – carbon heat. We can’t continue burning natural gas, and ‘green’ or ‘blue’ hydrogen won’t be here in any scale in the next decade (or two). Heat pumps, however, have a huge knock-on impact on how much extra upfront carbon we should spend on other measures to save heat, as saving heat energy won’t save much carbon in the 20 years of using a heat pump."

This all caused some discussion in the Passivhaus community, which is all about minimizing operating energy through the use of lots of insulation and triple-glazed windows. But as we keep saying, our problem today is not energy; we have lots of that. Our problem is carbon emissions, and if you look at the combination of upfront carbon and short-term operating carbon, there is a seductive logic to Banks' arguments.

There are also some issues that I raised with Banks. Firstly, whether this graph is believable. The British electrical system has been decarbonizing, but much of its so-called greening is due to the Drax power station burning biomass, mainly imported wood pellets. This is not counted as carbon emissions in the U.K. because burning trees is not considered fossil carbon, but if one is consistent about short-term carbon, then emitting CO2 from biomass now is not offset by trees growing for 40 years after. Banks admitted the point but noted that even if he adds back in the CO2 from Drax, the math still worked—that green line was just a bit steeper.

Then there is all that talk about green hydrogen going into the gas pipes; reading the British news, one gets mixed messages about where Britain is going. This alone might be a very good reason to concentrate on the building fabric and go Passivhaus; at least that is something that one can control and rely on now. You can't say that about British Prime Minister Boris Johnson and the British government.

There is also the concern I raised with the electrify everything gang: Where is all this green electricity going to come from? This is why we still need Passivhaus and e-bikes instead of heat pumps and e-cars—to minimize demand so that we have enough juice to electrify everything. The same concern was raised by Passivhaus architect Mark Siddall, who tells Treehugger:

"My concern is, short term optimisation which focuses upon a single point of reference, will have a negative, systemic and long term impact. For instance, as the grid becomes progressively decarbonised and we make the shift away from fossil fuels and toward a reliance upon renewable electricity we must be mindful that electricity is a costly energy source. Add to that the cost of inter-seasonal storage and we start to recognise the need to prevent the rise of fuel poverty."

Siddall also makes the point that we should be minimizing the amount of electricity we need, and the resources to make it.

"Of course it is not simply about affordability. There are broader issues that deserve consideration, such as resource efficiency. …each and every square meter of photovoltaic panel, each wind turbine demands resources and incurs an environmental impact. We are not simply facing a climate emergency. We are facing a crisis in biodiversity. This means, by optimising our buildings over the long term lifecycle, we reduce resources use and make a smaller imposition upon flora, fauna and wildlife in general."

The tweeter-in-chief at Mole Architects (known to Treehugger for Marmalade Lane cohousing) found it thought-provoking too, but like me and Siddall, worries about the electricity supply. But I do agree with Banks' tweet too—let's have an informed debate about this. And let's add "short term carbon" to our discussion about carbon literacy.

And, as architect Elrond Burrell reminds us, there is more to Passivhaus than just carbon.