Title: 2. How has climate changed? | Australian Academy of Science
Headings: 2. How has climate changed?
2. How has climate changed?
Past climate has varied enormously on a variety of time-scales
Past records demonstrate that global climate is sensitive to small but persistent influences
Box 2.1: How do we detect climate change?
Box 2.2: Has climate warming recently stopped?
Global average temperatures have increased over the past century
Changes are evident in many parts of the climate system
There are regional differences to climate change including within Australia

Content: Indeed, decades of little or no temperature trend can be seen throughout the last century, superimposed on the long-term warming trend. Two main factors have contributed to the most recent period of slowed surface warming. First, decadal variability in the ocean-atmosphere system has redistributed heat in the ocean, especially in the eastern and central Pacific. This has caused warming at depth and cooling of surface waters and the lower atmosphere in this region. Second, several temporary global cooling influences have come into play including unusually weak solar activity ( Box 3.1 ), increased aerosol production, and volcanic activity. None of these influences is likely to continue over the long term. Moreover, despite the slowdown in warming at the surface, there have been continuing increases in heat extremes and in the heat content of the oceans, as well as rising sea levels, shrinking Arctic sea-ice, and ongoing melt of ice sheets and glaciers. Some models predict that, when the current slowdown ends, renewed warming will be rapid. Scientists use ice core samples to reconstruct climatic records over hundreds of thousands of years. by NASA/Lora Koenig
Global average temperatures have increased over the past century
Climate and sea level were relatively stable over thousands of years of recorded human history up to the 19th century, although with some variations (Figure 2.1b).