Climate change has aptly been described as humankind’s greatest moral and technical challenge. A recent World Bank report predicts that more than 800 million people reside in areas that by 2050 will become dangerous climate hotspots (World Bank, 2018). If climate change causes a 3°C increase in average surface temperatures by 2100 (which is the expectation on our current trajectory) it is estimated to result in the extinction of 3 in every 5 species currently living (Flannery, 2008). Other studies (Thomas, et al., 2004), on the basis of mid-range climate-warming scenarios (temperature increases of 1.8–2.0°C for 2050, indicate that 15–37% of species in a representative sample of regions and taxa will be ‘committed to extinction’. At higher levels of temperature increase (>2°C) and where there are limited options for migration the extinction prediction increased to 38–52%. These dramatic extinction events are due to the relatively rapid rate of this change which does not allow these species to genetically adapt quickly enough. Loss of migratory pathways further prevents many from moving to compatible ecological zones at higher latitudes.

However, as emphasised by the World Bank report, presentation of the individual impacts of climate change do not adequately portray the full impact of climate change due to the “cascading” effects which are likely to be far more harmful than the immediate impacts. These cumulative impacts are difficult to accurately quantify. For example, from 2006 to 2011 Syria experienced its large-scale crop failures due to the worst drought in at least 900 years (a drought made 2-3 times more likely on account of only 0.6°C average temperature increase).

Figure 1 Current net emissions trajectory vs. that required to meet 2050 target of 1.5°C temperature rise (World Resources Institute, 2018)

This drought and the civil war have both contributed to the 6.6 million internally displaced persons and the mass migration of 5.6 million people to Turkey, Europe and elsewhere; which in turn contributed to Brexit, the rise of populist governments, and an unprecedented threat to the EU’s institutional survival (Glasser, 2018). The social and economic costs of this mass migration are, therefore, difficult to quantify. Likewise, it is impossible to predict the impact on humans of large-scale species loss, in a world in which we are intimately dependent upon plants, animals, insects and microbes for our food and oxygen; if their destruction alone is not enough cause for alarm.

Never before has an environmental phenomenon been scientifically studied as carefully and thoroughly as climate change. Yet its complexity and the existence of many positive feedback loops make it a difficult event to model and predict. Many scientists are expressing concerns that these positive feedback loops may lead to a tipping point causing climate change to escalate at such a pace that human efforts, no matter how strenuous, will no longer be able to halt its progress (Curtin, 2018).

Figure 2 Global warming trajectory: No change vs. Current commitment vs. Paris goal (World Resources Institute b., 2018)

As can be seen in Figure 3 below, New Zealand’s net emissions under the United Nations Framework Convention on Climate Change (UNFCCC) were 56.0 Mt CO2-e in 2016 and emissions per person were the sixth highest at 17.4 tonnes CO2-e per capita amongst developed countries (NZ MFE, 2018).Estimates of the global emissions outcome of current nationally stated mitigation ambitions (as submitted under the Paris Agreement) would lead to global greenhouse gas emissions in 2030 of 52–58 Gt CO2eq yr-1 (see Figure 1). Analysis reflecting these ambitions indicate they would not limit global warming to 1.5°C, even if supplemented by very challenging increases in the scale and ambition of emissions reductions after 2030 (IPCC, 2018). Instead our current trajectory is for a 2.7 - 3.7°C increase in average surface temperature (see Figure 2).


Figure 3 Comparison of per capita CO2 equivalent GHG emissions in 2016 (NZ MFE, 2018)

By planting 1 billion additional trees over the next 10 years New Zealand can expect to sequester approximately 450 Mt CO2 equiv. (author’s calculations) based upon the current target to plant two thirds native trees and one third exotics. This will buy only 8 years emissions at the current annual net emission rate of 56 Mt CO2 equiv. Even if New Zealand is able to reduce its existing annual emissions (in order to extend the benefit of these carbon offsets), these offsets (if in the form of commercial plantations) will themselves produce increased emissions in their maintenance, harvesting, processing and transporting. This is discussed in Section 5 below.

The Carbon Budget – Reaching Zero Emissions

While emissions reduction efforts (through reduced burning of fossil fuels and other GHGs) are a critical component of efforts to mitigate climate change; emissions reduction alone is insufficient to meet the 1.5°C target the IPCC claims are critical to achieve (IPCC, 2018). The 2018 IPCC SR15 report acknowledges that the 1.5°C target cannot be achieved without substantial capture and storage of carbon (CCS) which is already in the atmosphere through carbon negative technologies (i.e. not simply reduced emissions) (IPCC, 2018). Technologies for capture and geo-sequestration of atmospheric carbon are difficult and expensive and at best unlikely to provide significant impact until 2060 (IPCC, 2018). Currently the most viable method of biological sequestration of atmospheric carbon is through new forest plantations or reafforestation of previously cleared native forests. However, even if this was socially and politically achievable, there is simply not enough non-food producing land available to achieve this (Flannery, 2008).

Furthermore, bio-sequestration of carbon with terrestrial vegetation (commercial forestry) has problems with permanence. The carbon in three quarters (75%) of wood products is mineralised (returned to CO2) within 1 year of harvest; the remaining 25% of wood products last approximately 100 years (Ford-Robertson, 1996). With the exception of the approximately 25% of plantation wood that is used for durable wood products, and the limited quantities stored in soil, the only carbon stored by terrestrial plants is that which exists in the living plant. Most of the carbon in plant debris (lost branches and leaves) and processed wood of softwood species (the majority of commercial plantations), is quickly decomposed and returned to the atmosphere as CO2. For this reason, plantations must repay the full value of carbon credits associated with the trees harvested in forests participating in the Emissions Trading Scheme.

Carbon captured in terrestrial forestry also runs the risk of unintentional reversal as a result of fire or other destructive events.

Is Commercial Forestry a Solution?

While trees are currently our best and safest option for carbon sequestration they only provide a relatively short term solution. Furthermore, there is simply not enough available land area on earth to grow the number of trees required to reverse the carbon released from the post-industrial age extraction and burning of all the fossil fuels which have hitherto been locked away over millions of years underground.

Figure 4 Carbon budget for typical NZ radiata pine forestry plantation (Evison, 2017)

As forests grow they convert atmospheric CO2 into organic hydrocarbons. By about 34 years P. radiata plantation forests have reached a net zero sequestration equilibrium (Tee, Scarpa, Marsh, & Guthrie, 2012). Up to this point the forest is sequestering carbon and accruing carbon credits which can be sold through the Emissions Trading Scheme. At the point of harvest, forest plantations incur a carbon liability equal to the carbon units removed during harvest. As forests are repeatedly grown and harvested the amount of carbon stored peaks and falls as shown in Figure 4. However, the amount of carbon sequestered never increases beyond that of the first cycle when the forest reaches maturity at about 30 years of age. After this there is no net gain from carbon credit revenue. The economic benefit following the first cycle is only the net present value gain (due to the fact that carbon credit revenue is received in advance of the post-harvest carbon liability). That is to say, for forests commencing from bare land, after cycle 1, foresters participating in the ETS benefit only from a cash advance (in the form of carbon credit sales) against the liability to be repaid at time of harvest.