Query: When the PDO last switched to a cool phase, global temperatures were about 0.4C cooler than currently.

Relevant passages:
Pacific decadal oscillation <=SEP=> The Pacific Decadal Oscillation ( PDO ) is a robust , recurring pattern of ocean-atmosphere climate variability centered over the mid-latitude Pacific basin . The PDO is detected as warm or cool surface waters in the Pacific Ocean , north of 20 ° N. Over the past century , the amplitude of this climate pattern has varied irregularly at interannual-to-interdecadal time scales ( meaning time periods of a few years to as much as time periods of multiple decades ) . There is evidence of reversals in the prevailing polarity ( meaning changes in cool surface waters versus warm surface waters within the region ) of the oscillation occurring around 1925 , 1947 , and 1977 ; the last two reversals corresponded with dramatic shifts in salmon production regimes in the North Pacific Ocean . This climate pattern also affects coastal sea and continental surface air temperatures from Alaska to California .   During a `` warm '' , or `` positive '' , phase , the west Pacific becomes cooler and part of the eastern ocean warms ; during a `` cool '' or `` negative '' phase , the opposite pattern occurs . The Pacific Decadal Oscillation was named by Steven R. Hare , who noticed it while studying salmon production pattern results in 1997 .   The Pacific Decadal Oscillation index is the leading empirical orthogonal function ( EOF ) of monthly sea surface temperature anomalies ( SST-A ) over the North Pacific ( poleward of 20 ° N ) after the global average sea surface temperature has been removed . This PDO index is the standardized principal component time series . A PDO ` signal ' has been reconstructed to 1661 through tree-ring chronologies in the Baja California area .

Climate variability <=SEP=> Climate variability includes all the variations in the climate that last longer than individual weather events, whereas the term climate change only refers to those variations that persist for a longer period of time, typically decades or more. In the time since the industrial revolution the climate has increasingly been affected by human activities that are causing global warming and climate change.The climate system receives nearly all of its energy from the sun. The climate system also radiates energy to outer space. The balance of incoming and outgoing energy, and the passage of the energy through the climate system, determines Earth's energy budget. When the incoming energy is greater than the outgoing energy, earth's energy budget is positive and the climate system is warming. If more energy goes out, the energy budget is negative and earth experiences cooling. The energy moving through Earth's climate system finds expression in weather, varying on geographic scales and time. Long-term averages and variability of weather in a region constitute the region's climate. Such changes can be the result of "internal variability", when natural processes inherent to the various parts of the climate system alter the distribution of energy. Examples include variability in ocean basins such as the Pacific decadal oscillation and Atlantic multidecadal oscillation. Climate variability can also result from external forcing, when events outside of the climate system's components nonetheless produce changes within the system. Examples include changes in solar output and volcanism. Climate variability has consequences for sea level changes, plant life, and mass extinctions; it also affects human societies.