As countries negotiate a new international climate agreement for the post-2020 period—including at this week’s intersessional meeting in Bonn, Germany—the key choices for putting the world on a secure pathway to a low-carbon future should be front-of-mind. The new agreement will be essential for putting in place the policies beyond 2020 that ensure a shift from high-carbon to low-carbon and climate-resilient investments. To do this, the agreement will have to send the right signals to governments and businesses about the trajectory we need to be on.With that in mind, a central question facing the international community is: What will the time frames be for countries’ post-2020 emissions-reduction contributions?Now is the time to begin answering this question, as countries are required to propose their emissions-reduction contributions by March 2015. Getting the time frames right is essential to ensuring that the international climate agreement is both effective and ambitious. Should these contributions—which will begin in 2020—cover a period of five years to 2025? Or a period of 10 years to 2030? Or some combination? And should the agreement also include a long-term collective goal, such as phasing out greenhouse gas emissions by 2050?The answers to these questions hold significant consequences for the future of climate change action. It’s imperative that countries take them into account—both in Bonn this week and at future UNFCCC meetings.What Emissions Pathways Do We Need in the Short-, Medium- and Long-term?To consider the pros and cons of various time frames, it’s essential to keep in mind the emissions levels that allow us to meet the UN Framework Convention on Climate Change’s (UNFCCC) goal of keeping global temperature rise below 2°C (3.6°F). Exceeding this target would put the world at an increased risk of forest fires, extreme weather, and other climate change impacts.According to the U.N. Environment Programme’s recent Emissions Gap Report, global emissions in 2020 should be 44 GtCO2e on average for a likely chance of meeting the 2°C target. In 2025, emissions should be 40 GtCO2e on average and drop to 35 GtCO2e by 2030. (A decline of 5 GtCO2e is equivalent to eliminating emissions from all of the world’s cars, buses, and trucks in 2005). By 2050, emissions levels should fall to 22 GtCO2e in order to stay within 2 degrees of warming. For context, emissions in 2010 were roughly 50 GtCO2e.The scientific community is clear about what needs to be done by when, but what time frame for national contributions is best to put us on the right path and achieve the necessary emissions reductions?Based in part on research gathered for the GHG Protocol’s Mitigation Goals Accounting and Reporting Standard, we outline the pros and cons of a 2025 versus 2030 contribution time frame, as well as the possibility of an alternative that combines the two. In a later blog post, we’ll also look at the issue of a longer-term collective goal, such as for 2050.Pros and Cons of a 2025 End DateThe 2025 end date has a number of pros and cons. Some of the positives include:It can mobilize investment and planning for emission reductions, particularly for investments that operate based on relatively short investment horizons (such as some renewable energy investments). It can also encourage a quicker phase-out of inefficient practices and technologies.It also may be more consistent with the timing of some low-carbon development strategies (e.g. China’s Five Year Plans), and, relatedly, it could be easier to get buy-in from domestic constituencies, especially if the 2025 contribution is in line with commitments already made domestically. Accordingly, a 2025 end-date could provide greater certainty of targets being achieved given that they may build on efforts already underway to meet countries’ 2020 commitments and other existing policies.Importantly, the 2025 end-date provides more certainty of the emissions trajectory and, in turn, the carbon budget being used up. Short-term contributions can also help prevent emissions from rising continuously during the contribution period and then decreasing sharply in the target year.It avoids lock-in of low ambition targets for too long a period. If the level of effort Parties put forward for the 2015 agreement are not significantly ambitious, it may be better to have a shorter-term commitment while greater political will is built for the next round.However, a shorter time-frame is not without disadvantages:It may not facilitate longer-term investments for large, structural changes because the time frame may not be sufficient to include the lifecycle of large-scale capital investment projects.While a 2025 end-date could facilitate learning by doing through an evaluation of efforts, it may not be consistent with longer-term, low-carbon strategies.Further, it does not provide long-term guidance for future policymakers, nor guarantee a long-term commitment for emissions reductions. It could reopen negotiations prematurely, potentially distracting from implementation and posing risks to reaching agreement on a next set of contributions.Perhaps most problematic, a short-term time frame may not lead to significant additional ambition if policies and technologies already required for 2020 commitments continue to deliver substantial emissions reductions by 2025. And it may drive political leaders to commit to only modest levels of emissions reductions because they are the ones accountable to meeting those commitments (as opposed to more aspirational commitments on a longer-term time frame, when they will no longer be in office).Pros and Cons of a 2030 End DateA medium-term time frame could last a decade or so. In the context of post-2020 contributions, 2030 could be considered a medium-term time frame. Advantages and disadvantages of a medium-term time frame are in many ways the opposite of those of a short-term timeframe, but some unique aspects also come into play.On the plus side:A 2030 end date would better facilitate long-term planning for large, structural changes and capital investments and provide greater certainty for businesses and other stakeholders about the longer-term policy and investment context.It could mitigate the risk of unpredictable events, like an economic downfall, during any given year, making achievement of the commitment easier. And it may allow for more energy, time, and resources devoted to implementation, rather than continuous negotiations of short-term contributions.A medium-term time frame could also lead to greater ambition, as policymakers may be willing to make more ambitious commitments for dates farther in the future.However, there are a number of possible disadvantages of the 2030 time frame:It could encourage later phase-outs of less efficient practices and/or technologies, as it may not provide sufficient incentive for quick mobilization of investments if the emissions-reduction goals are considered to be too far in the future and not an immediate concern.It may also not provide sufficient detail in the short term for some investments. As a result, it may not encourage early adoption of efficient technologies that could yield greater aggregate emission reductions over the contribution period.A medium-term time frame also runs the risk of not being adopted by future political leaders. Additionally, it may not be consistent with the time frame of low-carbon plans and policies, which may span a shorter number of years.Importantly, it may not ensure a pathway that leads to significant annual reductions if reductions don’t occur until the very end of the time frame (e.g., if single year targets are adopted, such as with a target that limits emissions in only one year (e.g. 2030) as opposed to multiple years (2026-2030), Parties can make a steep decline in emissions at the end date of the target period but still contribute significant emissions levels during the period leading up until then. While this is still the case for a short-term time frame, there is less time for emissions to build up in the interim years.Best of Both Worlds? Coupled 2025 and 2030 ContributionsWhile both time frames have considerable advantages and disadvantages, one potential solution—taking the best of both approaches—is to encourage countries submitting both short- and medium-term contributions, one for 2025 and one for 2030. Given the strengths of each time frame, this approach could, in fact, be the likeliest to yield robust outcomes.There is precedent for the adoption of coupled targets, as some countries currently have both 2020 and 2050 goals. For example, as part of its domestic climate policy, the UK has embraced a series of short-term targets on the way to meeting a long-term goal of reducing its emissions at least 80 percent below 1990 levels by 2050. Coupled short-, medium-, and long-term contributions can help ensure that a certain emissions pathway is met, which can in turn limit cumulative emissions to the atmosphere over time.If Parties were to pursue coupled short- and medium-term contributions, Parties might decide to have one target be fixed and the other indicative. For example, a 2025 contribution could be the one that is more formally inscribed under the agreement, but the 2030 contribution is indicative of the emissions pathway of the country, with a provision for later adjustment if they are in a position make even more ambitious emissions cuts. This could have the advantage of providing investors with long-term signals, while also allowing decision-makers to adopt a more ambitious medium-term target than they otherwise would because there would be a longer period to put needed policies in place. The risk may be that politicians create distractions by putting forward ambitious indicative targets—which may be subject to change—and draw attention away from meager, short-term targets.Or one could take an alternative approach—making the 2025 contribution an indicative milestone along a pathway toward the 2030 contribution, which the Party is accountable for. The question that remains is whether Parties would be ambitious today with a 2030 contribution, especially considering that countries’ current emissions-reduction commitments fall far short of the necessary reductions for limiting warming to 2°C. Would a better outcome be reached if the global community negotiated the medium-term targets or revised them upwards at some point in the future if they are in a position to take more significant action?A third choice is for the 2025 and 2030 targets to have equal standing as fixed commitments, given the risks inherent in indicative targets. While Parties may not be as ambitious as they would be with an indicative contribution, there would be more certainty. And there could be a mechanism built into the UNFCCC that enables—and encourages—Parties to come forward with increased ambition over time if they are in a position to do so. The idea of a mechanism to continuously ratchet up ambition over time is currently under discussion in the UNFCCC negotiations and should be considered together with the time frames.Also, it is worth noting that while the spotlight regarding the post-2020 mitigation contributions is focusing on the 2025 versus 2030 timeframe, it is essential that commitments for even longer-term time frames be secured if we are to embrace a roadmap that will shift the world toward a low-carbon future. Indeed, significant dialogue has now begun on how to entirely phase out net greenhouse gas emissions by 2050, which has been found to be technically and economically feasible (watch for our forthcoming blog post on this topic).It is our hope that these considerations will be taken into account by negotiators when determining the next set of contributions. After all, this is the critical decade for preventing some of the most dangerous climate risks from being locked in for future generations.