By Ralph Ashton

It's possible. We know we can avoid dangerous climate change if we decide to.
The arithmetic of climate change mitigation centers on three main questions: How much greenhouse gas can we emit each year and still avoid dangerous climate change (the annual global carbon budget)?
Who gets to emit what portion of that annual global carbon budget? And who pays the financial and behavioral costs of changing from our current high-carbon global economy to the necessary low- or no-carbon economy? (Climate change adaptation is another important issue.)
The first question is straightforward. It is pure mathematics. No wriggling or clever rhetoric or complicated jargon or finger-pointing can change it.
To avoid the worst impacts of climate change, the world needs to stabilize greenhouse gas concentrations in the atmosphere at between 300 and 450 parts per million (ppm) of carbon dioxide equivalents (CO2e).
Analysis for Project Catalyst indicates that to stabilize at 450 ppm, humans can emit a maximum of 44 billion tons (gigatons or Gt) of CO2e in 2020 and 35 Gt in 2030, after peaking around the year 2015.
To put this in context, ""business as usual"" emissions are predicted to be 70 Gt in the year 2030. Many climate scientists now say that 300 ppm is the safe level, allowing an even smaller annual global carbon budget.
People often talk of reducing emissions by some percentage (25 percent, 40 percent, 90 percent) below the emissions in some base year (1990, 2000, 2005) by some target date (2020, 2050).
However, in plain speaking, we need to essentially decarbonize the global economy. And we need to do it pretty quickly.
The second question - who gets to emit how much - is a mix of what is fair and what is feasible. The fairness question is tricky and the real subject of the United Nations and bilateral climate change negotiations. It can be solved only by negotiation among countries.
The feasibility question is about what we can do in different countries in different sectors (industry, land use, and behavior) in what time frame. It can be influenced by policy but is determined more by technology and implementation constraints, and will change over time.
The third question - who bears the costs - is fairly obvious: The developed world needs to pay - because of its historical debt of causing most of the greenhouse gases currently in the atmosphere, because it has the financial and technical resources, and because it cannot reduce its own emissions rapidly enough to fairly deliver on the first two questions.
This is not about choosing between action in one country or sector over another. Or about letting countries or sectors ""off the hook"". Regardless of one's views on the fundamental issues of fairness, there are scientific and implementation constraints.
We humans simply have to do everything we can, everywhere we can, as soon as each action is feasible. And it is not about stopping development in developing countries. Those countries still have the right to develop. The international response to climate change should support them to do so on a low-carbon trajectory.
Any way you look at it, the climate solution will require concerted action from almost all countries, but especially the major emitting countries (whether developed, rapidly emerging, or developing).
It will require financial assistance from the rich world. It will require the sharing of technologies and experience between developed and developing countries - and among developing countries. It will require dramatic reductions in emissions from industrial processes.
And it will require the better management of terrestrial carbon (the carbon stored in land and vegetation) - recognizing that a numbing two-thirds of tropical forest carbon in developing countries is at risk of emission in the long term. Indonesia has a key role.
The climate solution needs to be implemented at a time when demand for food, fiber, fuel and land will reach its highest point since humans first walked the planet - with a global population growing from 7 to 9 billion by 2050.
In a dynamic world where putting land under ""protection"" for carbon sequestration will make land for other uses scarcer, increase the value of remaining land and commodities produced from it, and thereby impact on existing causes (the so-called ""drivers"") of land use change - including deforestation.
And at a time when demand for timber already exceeds the supply that can be met from legal and sustainable harvest.
It needs to be done in a world where slightly more carbon is stored in nonforested land and its vegetation than forested land in non Annex-1 countries.
And in a world where one country might be exhausting its forest resources just as another is beginning to harness its potential.
For example, 10 percent, or 33 Gt, of volatile forest carbon (carbon that would be emitted in the event of deforestation) is in countries classified as ""high forest, high deforestation"", the majority of which is in Indonesia. Conversely, 17 percent, or 56 Gt, is in countries classified as having ""low forest"" cover.
Land use (including forestry and agriculture) is currently the source of about one-third of all human-caused greenhouse gas emissions. It is also a staggering one-half of the feasible global solution in the years to 2020.
And it represents a full half of what developing countries can contribute to the climate change solution in the short to medium term. Avoiding dangerous climate change requires new incentives to better manage terrestrial carbon. To be effective, this must be done at scale.
As negotiators and world leaders continue to meet in Thailand this week for the penultimate climate change talks before the historic Copenhagen meeting in December, they must achieve more than they achieved at the UN High Level Meetings on Climate Change in New York and the G20 meeting in Pittsburgh over the past two weeks. They must demonstrate real progress toward effectively including terrestrial carbon in the solution through a mechanism that:
1. maximizes long-term terrestrial carbon volumes;
2. maintains existing terrestrial carbon and creates new terrestrial carbon;
3. includes all types of terrestrial carbon (using a phased approach starting with carbon and CO2 in peatlands, forest and lands that can become secondary forest);
4. uses a mix of complementary approaches (market and non-market, public and private);
5. takes action on terrestrial carbon in addition to, not in substitution for, deep reductions in greenhouse gas emissions from all other sources across the world;
6. recognizes sovereignty over land management;
7. builds appropriate national and international institutions;
8. avoids perverse outcomes; and
9. adapts to the best available information.
And Asia's leaders have the opportunity at the 15th ASEAN Summit, in Thailand in two weeks, to build further momentum on this crucial task through domestic action and regional cooperation, cognizant of the fact that 57 percent of the continent's standing tropical forest is under threat from deforestation in the long term.
The writer is convener and chair of the international Terrestrial Carbon Group, senior policy fellow at the H. John Heinz III Center for Science, Economics and the Environment, and visiting scholar at Columbia University. He can be reached at ralph.ashton@terrestrialcarbon.org.