If we’re going to curb the rise in greenhouse gas emissions, the world’s governments need to co-operate – and they’re running out of time to do it. That’s one of the top-line messages from the Intergovernmental Panel on Climate Change (IPCC)’s latest mega-report into current and future greenhouse gas emissions.

Today’s report is the last in a series of three from the IPCC. The first two summarised the physical evidence base for climate change and its potential impacts on ecosystems and human societies. Now the organisation is looking at what’s likely to happen to greenhouse gas emissions over the course of this century – and what possibilities there are for reversing the upwards trend.

The full report runs to thousands of pages, providing an overview of all the research in the area. In order to complete it, the scientists assessed more than 1200 scenarios for how the future might unfold from different studies. The report’s summary is slightly more digestible, at just 33 pages. Here’s our run-down of its key points.

Emissions rising

Between 2000 and 2010, greenhouse gas emissions grew at 2.2 per cent a year –  a faster rate of increase than over the previous three decades. Human-caused emissions were “the highest in human history” in the first decade of this century, the IPCC says.

And they’re slated to keep going up in the future. Under the scientists’ ‘business as usual’ scenario, emissions will surpass the limit associated with a temperature rise of two degrees Celsius above pre-industrial levels by 2030. The international community has agreed to hold temperature rise to two degrees in order to avoid “dangerous” human interference in the climate system.

Economic growth and rising populations are largely responsible for the rise – with the growth in the economies of poorer countries a particularly important factor. Without explicit efforts to reverse the rise, “emissions growth is expected to persist” in the future, the report says.

At this rate, the world could experience a temperature rise of somewhere between 3.7 and 4.8°C by the end of the century, compared to pre-industrial levels. This level of warming could lead to “severe, pervasive and irreversible” effects, according to the IPCC’s earlier  impacts report.

Predictions and promises

The worst impacts of climate change aren’t a given, however – because there is still time to stop the increase in greenhouse gas emissions reaching dangerous levels. It’s possible to limit temperature rise to below 2°C by the end of the century, the IPCC says.

But it would take a pretty epic effort – reducing global emissions by at least two fifths by 2050, and at least tripling or quadrupling the share of energy the world gets from low-carbon energy by the same date.

Emissions are rising so fast at the moment that it probably also means allowing concentrations in the atmosphere to ‘overshoot’ the safe limit, and then return to lower levels by the end of the century. In order to achieve this, we may need to use a new technology known as bioenergy with carbon capture and storage (BECCS), the IPCC suggests.

BECCS involves burning wood, plants or other crops, capturing the carbon dioxide that’s released, and then planting more. Because the new plants capture more carbon, it’s theoretically a carbon negative process.

But – like many other geoengineering techniques – BECCS is controversial, and as yet, there’s only limited evidence that it will work.

How to reduce emissions 

One big reason emissions are going up around the world is we’re using more and more energy – and generating increasing amounts of it from coal, which is very polluting.

Decarbonising the world’s power supplies is a crucial part of plans to reduce greenhouse gas emissions, the IPCC says. It’s likely to mean increasing the amount of power generated by nuclear power stations, renewables or fossil fuel fitted with carbon capture and storage technology, from 30 per cent of global supply at the moment, to more than 80 per cent by 2050.

Carbon dioxide emissions from air travel, trains and road traffic are projected to approximately double by the middle of the century – with increasing amount of transport used outweighing attempts to reduce emissions. Behavioural changes, technical solutions, changed transport patterns and changes to infrastructure could reduce this rise by 15 to 40 per cent, however.

Emissions from industry and manufacturing – also increasing rapidly – could be tackled by using materials more efficiently and by reducing overall demand for products. These measures would be “cost-effective, profitable and associated with multiple co-benefits”, the IPCC says. But few countries have managed to achieve them so far, so it’s not clear how they’re going to happen in practice.

In other areas change is already taking place. Some countries have introduced new low energy building codes as an effective way of reducing emissions – and significantly reduced the amount of energy used to heat or cool houses by retrofitting new technologies to old buildings.

Good news on forests?

This might all sound rather daunting. There is some good news, however. Over the last few years, deforestation rates have declined – and as a result emissions from agriculture, forestry and land use have probably also gone down.

The trend is likely to continue in the coming decades. Vegetation and the land could even be a carbon sink by the end of the century, rather than a source, the IPCC says – although if forests are adversely affected by climate change, then that might change matters.

Bioenergy could also play a critical role in reducing emissions. But this is only if important ecosystems like forests, grassland and peatland aren’t converted to create plantations for bioenergy products. Otherwise, large scale bioenergy deployment has the potential to increase emissions, instead of decrease them. The IPCC says there are “crucial issues to consider” on bioenergy, and scientific debates on whether it can reduce emissions aren’t resolved yet.

Costs and benefits

Limiting emissions to a level that would give a reasonable chance of avoiding a two degrees temperature rise this century will  entail global consumption losses – a measure of gross domestic product – of two to six per cent by 2050, according to the report. This equates to an annual reduction in consumption growth of 0.04 to 0.14 per cent a year, the IPCC says.

World economies will still grow, according to the IPCC – just by slightly less. The figures also don’t include the benefits of tackling climate change, or the savings that come from reducing the scale and seriousness of climate change impacts.

There are many co-benefits from cutting emissions, the report says. Shifting to lower carbon transport systems can create more access, mobility and safety for citizens, better health and greater energy security, for example.

Introducing policies

Countries around the world have introduced a variety of different policies and strategies with the aim of reducing emissions – from market-based systems, to reductions in subsidies for fossil fuels, city-wide climate action plans and regulations like energy efficiency standards. In 2012, 67 per cent of global greenhouse gas emissions were subject to some kind of national legislation or strategy.

But policies vary in their effectiveness. Cap and trade systems like the European Emissions Trading Scheme, for example, are being established in a growing number of countries. But, says the IPCC, “their short run environmental effect has been limited” because the caps that have been introduced have not been tight enough.

Climate change “has the characteristics of a collective action problem at the global scale”, the IPCC says – because most greenhouse gases accumulate over time and mix in the atmosphere – and tackling it therefore requires international co-operation. It adds:

“Effective mitigation will not be achieved if individual agents advance their own interests independently”.

Different countries have to work out how to share the costs and benefits of tackling climate change – a problem some have contributed to more, and others less, in the first place. This means that ethical considerations – including issues of wellbeing, justice and fairness – come into play in deciding on how they should be reduced. Countries are more likely to reduce emissions if the outcome they’re aiming for is seen to be fair.