By Luisa Cristini, PhD, University of Hawaii at Manoa
[Note from the editor: This is the thirteenth in a series of blog entries that will focus on introductory topics in climate dynamics and modeling, and will serve to provide insight into the current understanding of the science.]
In addition to the low frequency variability of glacial-interglacial cycles, more rapid changes have been observed during the past million years. Those that are best documented, in particular in Greenland ice cores, are associated with the millennium-scale variability that took place during the last glacial period. These variations are attributed to changes in the oceanic circulation and the oceanic heat transport, implying a large-scale shift in the climate. The deglaciation was also characterized by a strong millennium-scale cooling, called the Younger Dryas, which followed a period of warming that peaked around 14,000 years ago.
Mainly because of the influence of precession (the orbital parameter related to the distance between the Earth and the Sun at the summer solstice), the insolation (the solar energy received at the top of Earth’s atmosphere) was very different 10,000 years ago than it is now. In particular, the summer insolation at the North Pole was up to 50 Watts/m2 higher. As a consequence, the summer temperature in the northern hemisphere was relatively high during the Early Holocene and this period is often referred to as the “Holocene Thermal Optimum” or “Holocene Climatic Optimum”. However, the timing of the temperature maximum depends strongly on location, as during the Early Holocene, ice sheets were still present over part of Canada, inducing strong local cooling, and changes in oceanic and atmospheric circulation have a strong influence at the regional scale.
The last millennium is certainly the period in the past for which we have the greatest number of data. Tree rings, lake and marine sediments, ice cores, etc., all provide very useful information on past climate changes, but the amplitude and the exact time of the changes vary strongly between the different reconstructions. However, all the reconstructions reach their absolute maximum during the 20th century.
On these time scales, the two dominant natural forcings are changes in total solar irradiance (TSI), the electromagnetic energy emitted by the Sun that falls each second on 1 m2, and large volcanic eruptions. The majority of the volcanic eruptions have a dramatic local impact but only a weak large-scale influence on climate. By contrast, some major eruptions can transport large amounts of aerosols into the stratosphere where they can stay for a few years. Those small solid or liquid particles that float in the atmosphere modify its radiative properties, decreasing the solar irradiance at the surface and thus lowering the temperature, in particular in summer.
The last millennium is an ideal test case for climate models to compare natural and human induced changes. Whether driven by solar and volcanic forcings as well as by anthropogenic forcings (increase in greenhouse gas concentration, sulphate aerosol load, land use changes, etc.) the simulated temperatures are within the range provided by the reconstructions. This gives us some confidence in the validity of models. Furthermore, simulations can be used to analyze the causes of the observed changes.
On a regional scale, changes in the oceanic or atmospheric circulation can completely mask the influence of the forcing in some periods. As a consequence, the so-called “Medieval Warm Period” and the “Little Ice Age” can by no means be considered as globally or even nearly globally synchronous phenomena. The temperature in the first part of the second millennium was generally higher than in the period 1500-1900, but warm and cold periods occurred at different times in different locations. Analyzing the sources of climate variations on a regional scale is extremely complex because some changes in the circulation can be part of the response of the climate system to the forcing.
In the period 1906-2005, the global mean surface temperature rose by 0.75 ± 0.18°C. Moreover, the rate of warming increased sharply, with the increase in the last 50 years being almost double that in the last 100 years. This warming, which has lead to the highest mean temperatures in at least several centuries, is clear at global and hemispheric scale as well as over all the continents except Antarctica. The surface temperature of the oceans has also increased, although generally more slowly than that of the continents. The warming is associated with clear modifications of the cryosphere, such as a retreat of the large majority of glaciers, the permafrost and seasonally frozen ground, as well as a decline in the snow cover over land, especially in spring. In the Arctic, the sea ice extent has declined by about 3% per decade since 1978. The decrease in the extent of the sea ice is even larger in summer, at a rate of about 8% per decade. Over the period 1993–2003, the sea level rose at a rate of about 3.1 mm per year, the thermal expansion of the ocean and the melting of land ice (from glaciers and ice sheets) being the larger contributors to sea level rise.
When driven by natural forcings only, climate models cannot reproduce the observed warming. By contrast, if anthropogenic forcings are included, the results are compatible with the observed changes. The dominant anthropogenic forcing is the increase in greenhouse gas concentrations in the atmosphere. Humanity has also strongly affected the land use, in particular through agriculture and deforestation. The latter has an impact on the chemical composition of the atmosphere, for instance when the wood is burned and releases CO2. This also modifies the physical characteristics of the surface such as the albedo, roughness and water availability. While most of the human-induced changes in greenhouse gas concentrations and in sulphate aerosols have occurred in the last 150 years, land-use modifications started thousands of years ago in some regions, and certainly had an impact on climate at regional scale and perhaps at the global one.
The large changes in climate observed recently thus appear to be outside the range of natural variability on decadal to centennial timescales, but these changes are compatible with those predicted by models including anthropogenic forcings. This has led the IPCC to conclude, in its Fourth Assessment Report, that: “It is very likely that anthropogenic greenhouse gas increase caused most of the observed increase in global average temperature since the mid-20th century. Without the cooling effect of atmospheric aerosols, it is likely that greenhouse gases alone would have caused a greater global mean temperature rise than observed during the last 50 years. “Very likely” in this sentence means likelihood higher than 90%, while “likely” corresponds to the 66% level.
Because of the huge impact human society had and still has on the climate and Earth system, geologists have been considering the use of a new term to identify the period of human activity in Earth history. The term “Anthropocene”, in contrast to Holocene, was coined by the ecologist Eugene F. Stormerand and popularized by the Nobel-prize laureate Paul Crutzen, who considered the influence of human behavior on the Earth’s atmosphere in recent centuries so significant as to constitute a new geological era. The Anthropocene has no precise start date but, based on atmospheric evidence, may be considered to start with the Industrial Revolution (late 18th century). Other scientists link it to earlier events, such as the rise of agriculture. Evidence of relative human impact such as the growing human influence on land use, ecosystems, biodiversity and species extinction is controversial, and some scientists believe the human impact has significantly changed (or halted) the growth of biodiversity.
References and further resources
IPCC (2007). Climate Change 2007: The Physical Science Basis. Contribution of Working Group I to the Fourth Assessment Report of the Intergovernmental Panel on Climate Change [Solomon, S., D. Qin, M. Manning, Z. Chen, M. Marquis, K.B. Averyt, M.Tignor and H.L. Miller (eds.)]. Cambridge University Press, Cambridge, United Kingdom and New York, NY, USA. http://ipcc.ch/publications_and_data/ar4/wg1/en/contents.html
Zalasiewicz J., M. Williams, W. Steffen and P. Crutzen (2010). The New World of the Anthropocene. Environmental Science & Technology 44 (7), 2228-2231. http://pubs.acs.org/doi/full/10.1021/es903118j