This chapter examines how transatlantic travel writing helped to redefine climate throughout the long eighteenth century. Beginning with the challenge that early modern travellers’ encounters with North American extremes offered to classical theories of climate, this chapter first considers how fears about the inevitable influence of degenerative air gave way to a sense that settlers could change the climate. By emphasising the improving survival rates of European crops and animals, advocates for North American settlement reframed evidence of certain types of productivity as evidence that the climate itself was improving. By thus narrowing the definition of climate, however, this pattern of writing also opened a gap between the stories of improvement circulating in Britain and settlers’ reports on conditions on the (still cold) ground – and so, for many early modern observers, this transatlantic exchange also inspired new scepticism about the political interests served by promises of a changing climate.