Cloud Cover Feedback Moderates Fennoscandian Summer Temperature Changes Over the Past 1,000 Years

Northern Fennoscandia has experienced little summer warming over recent decades, in stark contrast to the hemispheric trend, which is strongly linked to greenhouse gas emissions. A likely explanation is the feedback between cloud cover and temperature. We establish the long- and short-term relationships between summer cloud cover and temperature over Northern Fennoscandia, by analyzing meteorological and proxy climate data. We identify opposing feedbacks operating at different timescales. At short timescales, dominated by internal variability, the cloud cover-temperature feedback is negative; summers with increased cloud cover are cooler and sunny summers are warmer. However, over longer timescales, at which forced climate changes operate, this feedback is positive, rising temperatures causing increased regional cloud cover and vice versa. This has occurred both during warm (Medieval Climate Anomaly and at present) and cool (Little Ice Age) periods. This two-way feedback relationship therefore moderates Northern Fennoscandian temperatures during both warm and cool hemispheric periods. Plain Language Summary Temperatures have increased globally over recent decades, strongly linked to increases in greenhouse gases. However, over Northern Fennoscandia summer temperatures have increased little over this period, although this region should be strongly affected by global warming. We suggest that changes in summer cloud cover, driven by global temperature changes, are responsible for this moderation of temperatures. This is happening now and during past episodes of climate change. We produce a new reconstruction of summer cloud cover for this region and compare it to existing temperature reconstruction to establish the relationship between temperature and cloud cover. We find that over short timescales, increased cloud cover leads to cooler temperatures and vice versa. However, over longer timescales (decades to centuries), we find that increased global temperature leads to increased northern cloud cover, which reduces local temperatures (the medieval period and at present). The opposite being true in globally cool periods, such as the Little Ice Age. These finding are important as they help to explain the feedback relationship between cloud cover and temperature, which is one of the major uncertainties in modeling future climate. Our data also confirm models of climate that suggest a poleward movement of storm tracks during recent warming.