Climatic Change (2010) 99:313–320 DOI 10.1007/s10584-010-9798-5 LETTER Postponing emission reductions from 2020 to 2030 increases climate risks and long-term costs A letter Michel G. J. den Elzen · Detlef P. van Vuuren · Jasper van Vliet Received: 13 October 2009 / Accepted: 2 January 2010 / Published online: 14 January 2010 © Springer Science+Business Media B.V. 2010 Abstract Substantially postponing the emission reductions, compared to the ranges indicated in IPCC’s recent assessment for 2020 as required for meeting the long- term 2 ◦ C target, increases the risk of exceeding this target. The costs of a delay strategy are lower in the short term, but leads to higher costs in the longer term. The analysis shows if the emission reductions are postponed to 2030 it is not likely that higher emissions from the earlier years can be fully compensated in future decades in a so-called ‘delayed action scenario’. A full compensation would require emission reduction rates in the coming decades that are much higher than those found in the scenario literature. Without compensation, the risk of exceeding the global temperature rise target of 2 ◦ C will increase. This confirms that it is not only the reduction commitments for 2050 that determine the risk of exceeding the 2 ◦ C target, but also the path between now and 2050. To meet this 2 ◦ C target, more ambitious 2020 reduction targets are needed for the developed and developing countries than those that have been pledged so far. 1 Introduction The EU, the Major Economies countries, and a large number of individual countries have agreed to a target limiting global temperature increase to 2 ◦ C, compared to pre- industrial levels. For this, greenhouse gas (GHG) concentrations in the atmosphere, in the long term, need to be kept below 450 ppm CO 2 eq to have a mean probability of over 50% of achieving this target (based on the uncertainty in climate sensitivity) Electronic supplementary material The online version of this article (doi:10.1007/s10584-010-9798-5) contains supplementary material, which is available to authorized users. M. G. J. den Elzen (B ) · D. P. van Vuuren · J. van Vliet Netherlands Environmental Assessment Agency, Bilthoven, The Netherlands e-mail: michel.denelzen@pbl.nl