Warmer conditions may be responsible for the increased export of carbon from peatlands to the oceans. A 65% increase in dissolved organic carbon (DOC) concentration in freshwater draining from upland catchments in the UK over the past 12 years has been observed. Rising temperatures may drive this process by stimulating the export of DOC from peatlands. Our results indicate that the flux of aged, riverine DOC of terrestrial origin, now recognized as a significant supplier of DOC to oceans, may increase substantially as a result of global warming. DOC concentrations have increased significantly (P<0.05) at 20 of 22 sites in the UK Acid Waters Monitoring Network. These sites span a wide range of acid-deposition levels, soils, topographies, land uses and geographical locations. Annual increases, averaging 5.4%, are proportional to mean DOC concentration (R²=0.81, P<0.001). As freshwater DOC concentrations are linked to storage of carbon in catchment soil, this indicates that increases are driven by regionally consistent processes within this carbon store, and that they are greatest at sites with large stores of soil carbon, such as peatlands. Although an inverse relationship has been proposed between mineral acidity and the generation of DOC, we observed similar proportional increases in DOC at remote, unacidified sites, as well as at those recovering from anthropogenic acidification. Changes in land use or river discharge do not account for the observed increases. However, the Central England Temperature Record shows that mean temperatures were 0.66°C higher in the 1990s than in the three preceding decades, and this factor could have influenced all sites. The enzyme phenol oxidase has been proposed to regulate carbon storage in peatlands. We therefore studied the thermal responses of peatland phenol oxidase in relation to the export of DOC. We subjected peat soil to a thermal gradient of 2–20°C. Phenol oxidase activity was greater at higher temperatures, although this enzyme is known to be highly constrained in these waterlogged soils. An increase of 10°C led to a 36% increase in activity (Q10=1.36). This was accompanied by an equivalent increase in DOC release (Q10=1.33) and an even greater increase in release of phenolic compounds (Q10=1.72) from the soil matrix. This selective enrichment with phenolic compounds is noteworthy because of the inhibitory character of these compounds. Under warmer conditions, selective enrichment should impair the metabolism of the remaining DOC, allowing even more DOC to reach the oceans.Warmer conditions may be responsible for the increased export of carbon from peatlands to the oceans. A 65% increase in dissolved organic carbon (DOC) concentration in freshwater draining from upland catchments in the UK over the past 12 years has been observed. Rising temperatures may drive this process by stimulating the export of DOC from peatlands. Our results indicate that the flux of aged, riverine DOC of terrestrial origin, now recognized as a significant supplier of DOC to oceans, may increase substantially as a result of global warming. DOC concentrations have increased significantly (P<0.05) at 20 of 22 sites in the UK Acid Waters Monitoring Network. These sites span a wide range of acid-deposition levels, soils, topographies, land uses and geographical locations. Annual increases, averaging 5.4%, are proportional to mean DOC concentration (R²=0.81, P<0.001). As freshwater DOC concentrations are linked to storage of carbon in catchment soil, this indicates that increases are driven by regionally consistent processes within this carbon store, and that they are greatest at sites with large stores of soil carbon, such as peatlands. Although an inverse relationship has been proposed between mineral acidity and the generation of DOC, we observed similar proportional increases in DOC at remote, unacidified sites, as well as at those recovering from anthropogenic acidification. Changes in land use or river discharge do not account for the observed increases. However, the Central England Temperature Record shows that mean temperatures were 0.66°C higher in the 1990s than in the three preceding decades, and this factor could have influenced all sites. The enzyme phenol oxidase has been proposed to regulate carbon storage in peatlands. We therefore studied the thermal responses of peatland phenol oxidase in relation to the export of DOC. We subjected peat soil to a thermal gradient of 2–20°C. Phenol oxidase activity was greater at higher temperatures, although this enzyme is known to be highly constrained in these waterlogged soils. An increase of 10°C led to a 36% increase in activity (Q10=1.36). This was accompanied by an equivalent increase in DOC release (Q10=1.33) and an even greater increase in release of phenolic compounds (Q10=1.72) from the soil matrix. This selective enrichment with phenolic compounds is noteworthy because of the inhibitory character of these compounds. Under warmer conditions, selective enrichment should impair the metabolism of the remaining DOC, allowing even more DOC to reach the oceans.