According to climate experts, including James Hansen, former lead climate scientist at NASA, the atmospheric concentration of carbon needs to be kept below 350 parts per million (ppm). However, the level has already reached 401ppm — and is rising by about 2ppm a year.

While there are uncertainties in climate prediction, according to the best scientific consensus we will need to limit warming to a rise of 1.5–2°C for a realistic chance of stabilising the climate. Since 1880, the planet’s average temperature increase has been 0.85°.

Without strong emissions cuts, scientists predict a 4° rise by 2100 and have warned that the increase could be as high as 6°. Either scenario would have very serious consequences for civilisation, including sea level rises, extreme heat waves, crop damage, species extinctions and loss of coral reefs.

The world’s soil holds more carbon than its total biomass and the atmosphere combined.

Focusing on catastrophe is ultimately unhelpful as a means of encouraging people to take action on climate change because it tends to induce a state of paralysis. A more optimistic perspective would be to look at whether it can be reversed rather than just slowed down.

Carbon capture & storage

One of the options for tackling climate change, carbon capture and storage (CCS), involves underground sequestration of carbon emissions from large industrial facilities such as coal-fired power stations. Results on the ground have so far been very limited; at present there are just eight operational industrial-scale CCS projects globally, and none is located in Australia or New Zealand.

As five of these eight projects sequester carbon to facilitate production in depleted oil fields, a process known as enhanced oil recovery, net carbon reduction benefits are thrown into question.

Limited to capturing new emissions rather than mopping up what is already in the atmosphere, CCS faces several hurdles. These include competition from the falling price of renewables, availability of suitable storage sites, long-distance transport infrastructure to the storage location and the risk that CO₂ could leak out. According to the US National Research Council, CCS carries a higher risk of causing earthquakes than fracking for unconventional gas.

Going carbon-negative

Today, few people seriously think CCS will play a central role in tackling the climate crisis, but the notion of going carbon-negative is increasingly entering the debate. Finding a means to suck the 51ppm of unwanted carbon out of the air is increasingly looking like a necessity. Several ways to help achieve this are already known and, while some are receiving major attention, it appears that others are being deliberately ignored.

At present, the most developed carbon dioxide removal technology is bioenergy combined with carbon capture and storage (BECCS). Biomass feedstocks are often considered to be close to carbon-neutral and, if the CO₂ emissions are stored underground, they are likely to become carbon-negative. However, in addition to the range of issues associated with CCS, large-scale BECCS represents a threat to native forests and land rights.

Perhaps we should be looking elsewhere.

Soil carbon

The world’s soil holds more carbon than its total biomass and the atmosphere combined. Like an enormous carbon bank, when soil erodes it loses carbon to the atmosphere. Alternatively, it can soak up atmospheric carbon while enhancing its fertility and water content, in addition to boosting food security and the economy.

Buying organic, consumers are more likely to be supporting soil carbon sequestration.

Most soils are in a worsening condition, and much of this is due to monocultures, tillage and a lack of cover crops. Soil carbon-storing systems are commonly no-till or utilise conservation tillage where at least 30 per cent of the previous year’s crop residues are left on the field when cultivating. Groundcover plants are excellent at preventing carbon loss. As the health of soils improves and they become carbon-rich, they develop a healthy spongy humus aided by the mycorrhizal fungi that occur in a symbiotic relationship with most plant species. Buying organic, consumers are more likely to be supporting soil carbon sequestration.

In Australia, soil carbon advocate Dr Christine Jones has identified carbon sequestration rates of up to 33 tonnes per hectare per year. This compares to more conservative figures from the soil carbon business CarbonLink of 2–10 tonnes per hectare per year when there is a shift from cropping to pasture. Accurate measurement systems over large acreages are difficult to achieve.

Unlike Australia, where the soils are arid and weathered, on the whole those in New Zealand already have a significantly higher soil carbon content, so there is less scope to pursue this strategy.

Carbon-negative food

Beef has a bad reputation for its sizeable ecological footprint, but grass-fed beef is very different from its feedlot cousin. Pastures that are managed under regenerative grazing regimes where animals are concentrated in a small space for short periods can be powerful carbon sinks, and the meat produced even has the scope to be carbon-negative.

A 2014 study by Yantai Gan in Nature Communications looking at wheat cultivation found that it can be carbon-negative, too. By using a carefully chosen range of techniques (fertilising crops based on soil tests, leaving the stem and roots in the soil at the end of the growing season, reducing summerfallow bare soil periods and rotating cereals with grain legumes), for each kilogram of wheat up to 380g of carbon was sequestered from the air. The follow-up question is whether this remains in the ground for an extended period.

Forests: not ideal carbon sinks

As the average tree absorbs around one tonne of CO₂ during its lifetime, reforesting the planet is commonly seen as a cure-all climate solution. Unfortunately, things are not so simple. Trees are slow to grow and carbon reductions that are achieved now are far more valuable than those that will occur decades down the track. After 20–50 years, trees usually stop absorbing carbon and can become net greenhouse gas emitters through avenues such as methane emissions from fallen leaves. Forests may be chopped down, attacked by pests or lost due to forest fires exacerbated by climate change.

So is timber itself carbon-negative? Some lifecycle analyses indicate it is if sustainable timber is used, coupled with renewable energy and energy efficiency at the manufacturing stage.

Hemp construction

Planting forests offers only a temporary carbon sink. We need something that gets the job done faster, with far less uncertainty, such as the right type of biomass.

Promoted as a wonder-crop, hemp is fast-growing, requires no pesticides, has modest water needs and enriches the soil. Hemp fibre represents roughly 1.4 times its weight in absorbed CO₂ and the ideal way to lock this away for an extended time is via construction. Options include in walls, roof and wall insulation, and as a fibreboard substitute. Even when the impact of other materials such as lime binders are factored in, a cubic metre of hemp wall stores about 110kg of carbon.

Switching from high-embodied-energy materials to hemp has a huge environmental benefit. A UK distribution centre built for the brewing company Adnams would have whacked the planet with 300–600 tonnes of carbon if conventional construction methods had been used. Instead, it used hemp and locked up 100–150 tonnes.

Industrial hemp is legal to cultivate in both Australia and New Zealand and is being used in a growing number of new dwellings and renovations.

Strawbale & bamboo

Another renewable material used for construction, strawbale can also be carbon-negative. Its super-insulated qualities are particularly suited to colder climates and it meets the Passive House standard for houses that maintain a comfortable temperature with no active heating or cooling.

In the UK, a company called Modcell has developed prefabricated panels made from a mix of carbon-sequestering materials including strawbale and hemp. DIY strawbale Home building in Australia and New Zealand is becoming more common, with regular workshops held and several straw construction companies. The eco-construction bible YourHome has technical tips for strawbale builders.

Also carbon-negative is bamboo, the fastest-growing plant on the planet. Left to its own devices, it reaches its capacity as a carbon sink after 5–8 years, after which point it requires managing via selective harvesting to remain a sink.

Bamboo is a mainstay of traditional homes in Southeast and East Asia. In Australia and New Zealand it’s used in construction as unprocessed poles, is sometimes made into panels as a plywood substitute and bamboo laminate can be used in flooring and furniture instead of hardwoods.

Biochar & gasification fuels

Plants and trees absorb atmospheric carbon while they are growing, and if they are burnt as biomass this is later released back, together with soot, which also contributes to climate change. Biochar production instead involves the use of pyrolysis (low-temperature combustion) to convert organic matter to carbon-negative charcoal.

A range of feedstocks can be used, including domestic organic waste and agricultural wastes that are in some cases produced in great quantities and often pollute the environment or are wastefully burnt.

Beef has a bad reputation for its sizeable ecological footprint, but grass-fed beef is very different from its feedlot cousin.

According to its supporters, biochar added to the soil in granular form has great benefits. They believe it can sequester carbon in the soil for hundreds to thousands of years and that it encourages additional soil carbon uptake, and point to studies showing increased yield. Biochar is sometimes “inoculated” with mycorrhizal fungi to boost its results.

Equally ambitious are moves to produce power in a carbon-negative way. Biochar can yield two fuels: bio-oil, which serves as a substitute for fuel oil, and hydrogen-rich syngas.

A type of biochar tabletop stove was created by researcher and inventor Robert Flanagan, and another, known as “The Beaner”, was formerly sold by a company called WorldStove. Gasification of coffee grounds and walnut shells was used to power a vehicle retrofitted by Chicken John, an eccentric who ran for mayor of San Francisco in 2007. At present, the only carbon-negative gasification fuel system on the market appears to be the Power Pallet, an industrial-sized 20kW electricity generator.

As with many other types of biomass, if biochar was scaled up sufficiently, experience suggests it would threaten forests and agricultural land, and risk “land grabs” in Africa and other continents. Activist group Biofuelwatch is critical of biochar for these reasons and also questions whether the hype stacks up.

Consuming carbon negative

While consumerism is generally viewed as a bad guy in environmental circles, is there a way of flipping it around by making products carbon-negative? A range of items are made from bamboo and hemp, for starters.

Methane is a powerful greenhouse gas with 28–84 times the global-warming potential of CO₂, and fugitive emissions are being captured by California company, Newlight Technologies. These are used to make a novel plastic with negative greenhouse emissions known as AirCarbon. A growing number of products are made from this material, including chairs, mobile phone cases and laptop packaging.

Hemp fibre represents roughly 1.4 times its weight in absorbed CO2, and the ideal way to lock this away for an extended time is via construction.

Carbon Capture Pak is a special type of packaging made especially for the green cleaning-product manufacturer ecostore in New Zealand. About 92 per cent of the raw material comes from Brazilian sugar cane byproducts, and the final product is identical to polyethylene, making it recyclable.

A lifecycle assessment has found that Carbon Capture Pak stores 2kg of CO₂ for every kilo of plastic used. As with AirCarbon, the carbon is locked in for the life of the plastic unless it is incinerated. This figure compares with the roughly 2kg of carbon generated from the manufacture of an equivalent weight of petrochemical plastic.

Beyond offsetting

A 2015 study led by Thomas Gasser in Nature Communications used computer modelling to conclude that keeping warming under two degrees will involve both highly aggressive carbon cuts and negative emissions. Any notion that carbon-negative techniques can justify business as usual at the pollution end can be dispelled.

If carbon removal initiatives are tied to emissions trading and the carbon market, the likelihood of seeing unsustainable industrial-scale projects is multiplied. At a time when CCS tends to dominate the carbon-negative debate, some carbon removal solutions might not be as complicated as we imagine.