By Chris Johansen

Greenhouse gas offsetting refers to an action that compensates for the human-induced emission of carbon dioxide or other greenhouse gases (GHG) to the atmosphere.

The most obvious example is the planting of trees, or other forms of revegetation, which absorb atmospheric carbon dioxide through photosynthesis.

Other more dubious examples include desisting from previously intended tree felling or other vegetation removal, installing renewable energy, increasing energy efficiency and sequestering carbon in the soil. 

Australia’s carbon credits scheme, which issues Australian Carbon Credit Units (ACCUs) for a given amount of carbon drawn down or abated, has recently been described as a “rort”. So described by former insiders of government agencies responsible for integrity of ACCUs – the Climate Change Authority and Emissions Reduction Assurance Committee – and CSIRO experts on vegetation growth. 

The arguments against the current operation of the ACCU scheme seem plausible to me but I have deeper concerns about the philosophy of offsets themselves.

My main concern about offsets is that they are invariably used to justify ongoing, and even increased, GHG emissions. That is, use of offsets effectively delays the reduction of GHG emissions, which inevitably delays action on climate change, an action we can no longer afford to delay.

Some offsetting methods are clearly fraudulent. Firstly, with regard to desisting from previously intended vegetation removal. It is highly subjective, and hardly verifiable, to claim that vegetation was not being removed due to climate change concerns, and thus open to abuse.

Other more plausible reasons to keep vegetation intact include biodiversity concerns, a decision that land clearing for agriculture was uneconomic or due to advocacy by First Nations people.

Secondly, installation of renewable energy and increasing energy efficiency now proceeds primarily for economic reasons – it saves money in the medium to longer term – and is not reliant on financial support from GHG emitters.

It is likely to proceed in any case and is therefore not compensating continued GHG emissions by any particular emitter. If an emitter replaces their own GHG emissions by switching their operations to renewable energy then that is to be encouraged – but this process is replacement and not offsetting. 

On the surface, compensation of GHG emissions by growing trees or other vegetation seems fine. But there are some problems here also, related to time scales. A calculation is made of how much carbon would be sequestered by the vegetation over a time period of 20-30 years. This determines the quantum of GHG emissions to be offset.

However, these emissions are usually either recently released or likely to be released in the near future. Considering the growth curve of trees over a 30 year period, only 10-20% of the carbon is likely to be sequestered in the first 10 years and the bulk of it in the last few years.

That is, the actual offsetting is postponed well into the future, well beyond the current critical decade when net GHG emissions need to be drastically reduced.

Calculation of forest growth curves over time is not so easy as they are highly specific to species, climate and soil type. It is not a simple matter of extrapolating from previously established growth curves. This is further complicated by the onset of climate change that we are now witnessing. 

Temperature and rainfall conditions are changing from the historical record, often in ways that are difficult to predict. Small changes in these conditions can have large effects on tree growth patterns.

Further, changing climate conditions can shift the balance of pests and diseases to which the given vegetation may become vulnerable, usually in an unpredictable manner.

Rising temperatures with increased thunderstorm activities can initiate wildfires, releasing much of the carbon dioxide previously sequestered.

To guarantee that a given quantum of emissions will be compensated by forest growth decades into the future is becoming an increasingly uncertain proposition.

Another means of claiming offsets is through build-up of carbon in agricultural soils. This can be achieved through such techniques as regenerative or conservation agriculture, whereby maximum crop residue and animal excreta are returned to the soil and the soil is not ploughed.

Having measured soil carbon in various countries over the past half century I can testify as to the difficulties of quantification of soil carbon to the extent required to equate it to offsets. 

Firstly, soil carbon concentrations are spatially highly variable, depending on where a clump of residue or dung actually fell. Therefore, many soil samples are needed per paddock to get a realistic assessment of the carbon status of the paddock, and any changes over time.

Secondly, this soil carbon is quite ephemeral – carbon dioxide readily returns to the atmosphere depending on whether the soil is ultimately ploughed, temperature and moisture status, microbial activity, etc.

In summary, to realistically quantify a build-up of soil carbon over time would require a multitude of soil analyses, beyond the affordability of most.

I thus conclude that the use of GHG offsets to combat climate change is a furphy. Indeed, it provides an excuse for continued GHG emissions. Efforts should instead be focussed on direct reduction of those emissions, for which economically viable technological options are available.

Reintroduction of a carbon tax would be the most effective way to encourage emitters to reduce their emissions, rather than prolong them via offsets.

Of course, carbon sequestration options, like revegetation, should be vigorously pursued as carbon drawdown will be required into the indefinite future. But this needs to be decoupled from GHG emissions reductions activities so as not to provide excuses for delaying emissions reduction.