Branched glycerol dialkyl glycerol tetraethers (GDGTs) are complex lipids of high molecular weight, recently discovered in soils and produced by still unknown bacteria. They are increasingly used as paleoclimate proxies. Their degree of methylation, expressed in the MBT, was shown to depend on mean annual air temperature (MAAT) and to a lesser extent on soil pH, whereas the relative abundance of cyclopentyl rings of branched GDGTs, expressed in the (CBT), was related to soil pH. Northern peatlands contain approximately one third of the world’s organic carbon and may play an important role in the responses of the global carbon cycle to climate change. The aim of this work was to study the effects of experimental climate warming on the abundance and distribution of branched GDGTs in a Sphagnum-dominated peatland (French Jura Mountains). Air temperature was experimentally increased using a warming system consisting of in situ open minigreenhouses (Open-Top Chambers – OTCs). The effect of the OTCs was especially apparent in spring and summer, with an increase of mean and maximal air temperatures of ca. 1 and 3°C respectively. Branched GDGTs either present as core lipids (CLs; presumed of fossil origin) or derived from intact polar lipids (IPLs, markers for living cells) were analysed. Results showed that despite the short duration of the climate experiment (26 months), branched GDGT distribution was significantly affected by the temperature rise, supporting the empirical relationship between MBT and MAAT established from a large range of soils. The difference in branched GDGT-derived temperatures between the control and the OTC plots was in the same range as the increase in maximal temperature induced by the OTCs in spring and summer, suggesting that branched GDGT-producing bacteria might be more active during the warmest months of the year. The OTC treatment had no significant effect on the abundance of branched GDGTs, mainly present as “fossil” CLs (70 to 85% of the total extractable branched GDGTs). Furthemore, no significant differences in branched GDGT distribution were observed between CLs and IPLs, which both provided higher MBT and MAAT values for the OTCs. This suggests that the fossil pool of branched GDGTs has a very fast turnover (less than the 2 year duration of the experiment) at peat surface and that branched GDGT distribution may rapidly reflect changes in environmental conditions. This work was funded as part of the PEATWARM initiative through an ANR (French National Agency for Research) grant (ANR-07-VUL-010).