What role has human activity played in causing climate change

Bring fact-checked results to the top of your browser search. Not only do they affect such geographic variations, but they also influence temporal changes in climate. The climatic modulations that occur at shorter scales include such periods as the Little Ice Age from the early 14th to the midth centuries, when the average temperature of the Northern Hemisphere was approximately 0. Several climate fluctuations on the scale of decades occurred in the 20th century, such as warming from tocooling from toand the warming trend since

The actual temperatures that characterize a cold event vary regionally and seasonally, but the temperatures during such an event will be in the cold tail of the probability distribution of temperatures for a location or region and time of year. The event definitions most often are based on daily temperatures, although multiday or longer averages also have been used.

The criteria can be either an absolute temperature threshold e. Cold outbreaks ranging from days to seasons still occur, but their frequencies and magnitudes are decreasing. Page 87 Share Cite Suggested Citation: The National Academies Press. Metrics of duration can be the length of time e.

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In some instances, the severity of a cold event has been quantified as the product of the event duration and intensity. Prior Knowledge and Overview of Attribution Studies Extreme cold events are driven by a combination of thermodynamics cold air mass formation and dynamics the large-scale circulation, advection.

Horton and colleagues have used self-organizing maps derived from atmospheric reanalyses to show that both factors have played roles in recent changes in extreme cold events. In particular, increasing trends in northerly flow have led to an increasing trend in winter cold extremes over central Asia.

The research to date indicates that extreme cold events are less frequent and less severe than in previous decades, although interannual variability is still large enough to allow extreme cold events such as occurred in North America in and Europe in Even over year periods, trends in the coldest temperature of the year are not compellingly positive over Europe and the United States van Oldenborgh et al.

The increases in cold extreme daily minimum temperatures i. The general expectation is that cold events defined relative to fixed temperature thresholds should become less frequent and less severe as the climate warms on the global scale. But, it is nonetheless possible for them to increase in frequency or intensity regionally for periods of time e.

Extreme cold events in eastern North America have characterized a few recent winters, but such events are less frequent and their actual temperatures Page 88 Share Cite Suggested Citation: In an analysis of observational data, van Oldenborgh and colleagues find that the return times of the lowest minimum temperatures of in the midwestern United States ranged from 6 to 44 years in the present climate, but only from 3 to 7 years in the climate of the s; likewise, return times of the cold winter-averaged temperatures were greater in the present climate than in the s.

Decreases in cold wave events of 4-day duration had the lowest frequency during the decade in all eight subregions of the United States examined by Peterson and colleagues balthough the decade of the s had the highest frequencies nationally. There is a notable absence of conditional attribution studies pertaining to extreme cold events.

Nevertheless, observational studies do provide evidence of a general decrease in the frequency of occurrence of extreme cold temperatures over the past few decades in most land areas of the world Hartmann et al.

Kharin and colleagues show that the trends of extreme cold ETCCDI indices are comparable in atmospheric reanalyses and CMIP5 historical simulations in which external forcing was historical.

In this respect, external forcing including its anthropogenic component is implicated in the decreasing frequency of observed cold extremes. The reduction of cold extremes has been detected and attributed in extreme seasonal and annual temperatures Christidis et al.

Attribution studies by Kharin and colleagues and others have drawn on comparisons of observational data with climate model simulations driven by natural and anthropogenic forcing. More recently, Wolter and colleagues also find decreasing frequencies of extreme cold events: The decreased frequency of cold extreme arises primarily from the underlying increase of the mean temperature, not from the decreased variability Screen et al.

The fact that underlying warming has moderated cold extremes also has been shown using daily circulation analogs for the European cold events of Cattiaux et al.

Page 89 Share Cite Suggested Citation: By prescribing reduced Arctic sea ice cover but historically observed ocean temperatures outside of the Arctic in two different global climate models, Screen and colleagues find that ice loss is associated with decreased likelihood of extreme cold events as well as decreased variability of temperature over nearly the entire Northern Hemisphere land areas.

The exception is the central Asian region, where the probability of extreme cold events increases with ice loss, in agreement with earlier studies Inoue et al.

For the rest of the hemisphere, the underlying warming dominates the trend of extreme cold events, implying that thermodynamically induced changes dominate dynamically induced variations, such as the jet stream.

While some studies do point to influences of sea-ice change on large-scale dynamics Francis and Vavrus, ; Jaiser et al. Additional attempts to link Arctic warming with an amplified jet stream and cold winters in middle latitudes have been made by Francis and Vavrus On the Horizon While the observational network is sufficiently dense to capture extreme cold events over most land areas except possibly Antarcticathere have been few evaluations of the ability of models to simulate the frequency and the intensity of these events.

Sillmann and colleagues and Whan and colleagues show that some climate models are able to capture the linkage between atmospheric blocking and cold events over Europe and North America, respectively.

The lowest temperatures are often reached under clear-sky, calm conditions characterized by strong near-surface temperature inversions. Limited vertical resolution is likely to impact model simulation of temperatures in such situations. It also is apparent from the studies cited above that atmospheric blocking events must be well simulated if models are to simulate extreme cold events realistically.

Finally, decadal and even longer trends in cold extremes can be impacted by multidecadal variability in the climate system e. With regard to a possible role of sea-ice loss and Arctic amplification, mechanistic linkages are still an active area of research.

Hypothesis-driven model experiments are needed to identify any dynamic mechanisms linking Arctic changes with midlatitude extreme events.Read chapter 4 Attribution of Particular Types of Extreme Events: As climate has warmed over recent years, a new pattern of more frequent and more intense.

Dec 15,  · Climate had a major role in the rise and fall of Roman civilization. Four scientists have won prestigious medical awards for genetics research and development of a widely used anesthetic nicknamed "milk of amnesia.". ENCYCLICAL LETTER LAUDATO SI’ OF THE HOLY FATHER FRANCIS ON CARE FOR OUR COMMON HOME.

1. “LAUDATO SI’, mi’ Signore” – “Praise be to you, my Lord”. Scientific opinion on climate change is a judgment of scientists regarding the degree to which global warming is occurring, its likely causes, and its probable consequences.. A related—but not identical—term, "scientific consensus on climate change," is the prevailing view on climate change within the scientific community.

The consensus is that: Earth's climate has warmed significantly. ENCYCLICAL LETTER LAUDATO SI’ OF THE HOLY FATHER FRANCIS ON CARE FOR OUR COMMON HOME.

1. “LAUDATO SI’, mi’ Signore” – “Praise be to you, my Lord”.