Original Policy Research at Hammerschlag LLC

Possibly the most underrated tool available toward mitigating global warming, is increasing the albedo (reflectivity) of the built environment on Earth’s surface. Brightening the colors of roofs, parking lots and roads induces global cooling by reflecting a greater fraction of incoming solar radiation back into space. The effect is nontrivial. For example, residential homes retrofitted with a more reflective “cool roof” to reduce air conditioning energy, induce a direct global cooling effect that is three to seven times greater than the indirect global cooling effect of avoided air conditioning energy.

Albedo management invokes a rich spectrum of technologies and techniques, a few of them as easy and cheap as choosing a paint color. Yet, there are almost no policy mechanisms in place to reward albedo management, either socially or financially. Voluntary and regulatory greenhouse gas (GHG) reductions are traded in new but quickly maturing markets that offer prices between $10 and $200 per metric ton of carbon dioxide equivalent. Albedo increases should be eligible for equivalent monetization. Hammerschlag LLC is researching and advocating innovative, new policy mechanisms that allow this. We believe that the lowest-friction pathway to get there may be to make albedo increases fungible with greenhouse gas reductions. This way, albedo projects can simply “plug in” to the existing, developing markets for GHG reductions.

The GWP of Albedo Change

If you are reading this, then you are probably already familiar with the widespread use of global warming potential (GWP) to cast different GHGs in units of carbon dioxide equivalents (CO₂e). For example, one ton of methane has GWP 25: one ton of methane means 25 tons of CO₂e.[2] Besides allowing scientists to compare different gases physically, GWP also allows traders to make different gases fungible in GHG reduction markets. A trader selling 25 tons of avoided GHGs from an industrial efficiency project, will fetch the same money as a trader selling 1 ton of methane destroyed at a dairy farm. So what’s the GWP of an albedo change? If we can decide what the value is, then we can trade albedo change in the same markets as GHG reductions.

It turns out the math is not very hard to do. GWP is computed by comparing radiative forcing integrated over an agreed-upon timeframe (usually 100 years). Radiative forcing is the solar energy received at Earth’s surface minus the energy radiated into space from Earth’s surface. It is reported in units of Watts per square meter (W/m²) and in a stable climate it is zero. Human-induced GHGs are causing the energy emitted to be smaller than the energy received, so at this moment Earth is subject to a radiative forcing of about 3.0 W/m². This radiative forcing is the cause of global warming. The GWP of each GHG is computed by dividing its radiative forcing by the radiative forcing of an equal quantity of CO₂.

Albedo has an even more direct relationship to radiative forcing: when the Earth’s surface is made more reflective, more of the received sunlight is simply reflected back into space. The incoming solar energy doesn’t even have a chance to add heat to the climate system. Incoming solar energy is measured in units of W/m² to begin with, so calculating a change to radiative forcing from a change to albedo is simple arithmetic.

Provisional Definition of Albedo Forcing Potential (AFP_.01)

Hammerschlag LLC is choosing to advocate and work with a provisional equivalent to GWP that we are calling Albedo Forcing Potential (AFP). We’re not simply calling our metric “GWP,” because that term is now the subject of a very large body of literature comparing GHGs amongst each other. GHGs do impact radiative forcing through an entirely different mechanism than albedo, so it seems wise to keep that physical difference visible with a different name.

Policymakers have converged on one metric ton of CO₂e as the de facto unit for goal setting, lawmaking and trading. Albedo policy will also need to converge around a common unit, if discussions and laws are to be coherent. Hammerschlag LLC proposes that this unit be a 0.01 albedo decrease (darkening) over a surface area of 1 m². The area of 1 m² matches conventional units of radiative forcing, is commonly understood, and relates well to measures of the urban projects that will make up the bulk of marketable albedo increases. A 0.01 increment in albedo is intuitive because it is equivalent to one percentage point when albedo is expressed as percent of solar energy reflected. 0.01 is also roughly the limit of engineering control we have over albedo: eventual weathering and dirtying make specification of albedo with any more precision unreasonable. To make the standard albedo increment of 0.01 transparent, we add this as a subscript to the metric, making it AFP_.01.

It may be counterintuitive to define AFP_.01 as a darkening, since this is the opposite of what we are trying to do with climate policy. An albedo increase is an environmental good, but a GHG increase is an environmental bad. Because GHGs are an environmental bad, GHG markets assign positive financial value to negative values of GHGs (GHG reductions). To make albedo as easily fungible in the market as possible we make its standard unit an environmental bad as well, by defining it as a darkening rather than as a brightening. This way, traders will not be wresting with opposite-signed products and the inevitable accounting errors that would result.

Policy Challenge: The Relationship of AFP_.01 to Time

The relationship between AFP_.01 and time is complex. Understanding and accounting for this complex relationship to time will be the principal challenge of creating policy that makes albedo increases fungible with GHG reductions. There are three domains of time-sensitivity.

1. Drift in Project Albedo

Over the course of a project’s lifetime, the albedo may change. In the case of cool roofs, the primary such change is due to weathering. Over time, degradation in the roofing material and accumulation of dirt and detritus can both decrease the albedo. Maintenance and cleaning ameliorate this effect, but of course the timing and quality of the maintenance and cleaning events induce their own, poorly predictable variance over time.

2. Project Duration

If albedo at a project site returns to its pre-project value, there is an instantaneous loss of the project’s radiative forcing change to the climate system. In contrast, once a GHG emission has been avoided, the climate impact of the avoided emission persists through the 100‑year time horizon, to whatever degree the unavoided GHG emission would have persisted through the 100-year time horizon.

The AFP_.01 metric is only accurate to the extent that the albedo project lasts as long as the time horizon. Possible remedies for this limitation include:

• Shorten the GWP time horizon to a period commensurate with typical albedo project length;
• Compute project-specific AFP_.01 values, that mathematically account for variable albedo within the time horizon;
• Use a standardized AFP_.01 value, but prorate project climate credits according to the fraction of the time horizon covered; or
• Deploy policy changes that promise persistence of the albedo change throughout the time horizon.

3. Decay of the CO₂ Reference Pulse

The 1 kg CO₂ reference pulse decays substantially during the 100-year time horizon. This means that the reference radiative forcing is not a constant, so computation of AFP_.01 can and should relate to the difference between the albedo project and the reference pulse’s radiative forcing over time. This particular time effect does not need policy attention per se, but understanding it is critical to developing a physically meaningful mathematical formulation for AFP_.01.

Policy Challenge: the Relationship of AFP_.01 to Location

The impact of albedo to radiative forcing is different in differing local circumstances. Each of the following can and does have an effect:

• Latitude. Albedo changes at very high latitudes will have a smaller effect per unit surface area than at lower latitudes.
• Cloud cover. Albedo changes in sunnier climates will have a greater relative effect.
• Aerosols/pollution. Any substance that absorbs shortwave radiation between the Earth’s surface and the top of the atmosphere, reduces the climate impact of albedo changes.
• Shading. Trees, hillsides, neighboring buildings, or other structures that cast shade on the roof will reduce the impact of albedo change.
• Snow cover. Climates that experience substantial snow cover each year will produce smaller effects from albedo changes in the built environment.

Recognizing these local effects requires either computing AFP_.01 on a project-specific basis, or establishing project correction factors that adjust CO₂‑equivalents computed with a generalized AFP_.01. Hybrid solutions are possible, for example a set of semi-generalized AFP_.01 values might be established for multiple latitudes, and the remaining location-specific parameters handled as correction factors.

Moving Forward

The first step will need to be authorship of a methodology acceptable in the voluntary carbon market. Hammerschlag LLC is currently working on doing just this. Contact us if you are interested in contributing. We need help from influential policymakers, traders, and scientists to give albedo management the light of day, as another important tool in fighting climate change.

[2] 25 is the value given in the Fourth Assessment Report issued by the Intergovernmental Panel on Climate Change (IPCC) in 2007. In 2014 the IPCC released the Fifth Assessment Report which elevated the value to 28. Though the more recent science supports this higher value, the California climate policy mechanisms cited here utilize the value of 25 for the sake of internal consistency and, once again, to encourage fungibility in a stable market.