What is climate?
First, it is important here to note the differences between weather and climate. Weather is local and what you can see out of your window; weather is the cold British winter snows in 2011 and our 2012 wet and rainy summer (these are to do with short term variations in winds and atmospheric pressures), or heat waves in the USA in July 2012. Climate is much broader scale; we are looking at long term statistical patterns in weather. An individual flood or storm is not caused by climate change, but climate change may make extreme weather events (like floods) occur more frequently. “Climate” encompasses data such as temperature, humidity, pressure, wind, precipitation (snow or rain), and other meteorological measurements.
Throughout the last 2.6 million years (the “Quaternary Period”), the earth’s climate has oscillated many times, swinging between glacial and interglacial states (Figure 1). Over the last ~1 million years, we have experienced large ice ages and interglacials with a periodicity of around 100,000 years. We are currently in an interglacial state, which began at the start of the Holocene, ~11,500 years ago. About 104 stages of these cold and temperate cycles have been recognised in deep ocean marine sediment cores (Figure 1) . During glacials, large ice sheets developed in mid- to high-latitudes, including over Britain and North America. These large changes are driven by changes in the earth’s orbit around the sun – see The Quaternary Period (Table 1) . Glacials and interglacials can be further divided into stadials and interstadials, and within these we have smaller scale Dansgaard-Oeschger cycles, and then even smaller cycles, such as El-Nino and ENSO. Climate data is therefore very noisy, and climate scientists must determine patterns in this data using complex statistical techniques. Throughout this time, carbon dioxide has mirrored temperature variations, which have formed a regular pattern.
Climate variations are a natural part of the earth system. It is therefore important, using instrumental records and proxies (such as ice cores, or microfossils in marine sediment cores), to compare current trends with those in the past.
Is our climate changing?
The Earth warmed by an average of 0.6 ± 0.2°C during the twentieth century . Rapid warming has been measured with global instrumental data since the 1800s (Figure 2). The 2001 IPCC report stated that most of this warming was likely to have been due to an increase in greenhouse gas emissions. However, this average rate hides considerable variations in the rate and magnitude of warming. Climate change varies seasonally, on decadal timescales, and is geographically patchy. Three areas in particular have been subject to recent regional rapid warming (sensu Vaughan et al., 2003), with rates of warming far faster than the average noted in the IPCC. These regions are: north-western North America, the Siberian Plateau in northeast Asia, and the Antarctic Peninsula and Bellingshausen Sea. These areas warmed by more than 1.5°C between 1950 and 2000 AD (compared with a global mean of 0.5°C).
More populated regions have atmospheric sulphate aerosols, that may mask warming. Urban meteorological stations may also record anomalous warming due to urban heat island effects. However, the Antarctic continent is free of these effects. Although direct meteorological observations are short (~60 years), trends in these areas are particularly important.
The Longer Term View
Although we have measured changing temperatures and carbon dioxide levels at short timescales (since the 1700s), surely this could just reflect natural variability over short timescales? In order to understand our climate, it is very important to look at the long-term view.
In many places, atmospheric temperatures are now warmer than they have been throughout the Holocene. Figure 3 shows that the temperature of the last 200 years is much higher than it was when Romans were making wine in southern England. However, what is really concerning is the rate of change. Temperatures have risen far more sharply in the last century than they have at any point in the last 2000 years.
This conclusion is reached again and again, and a paper by Mann et al. (2008) in PNAS  show that the Earth’s temperature is anomalous in a long-term time context. Mann et al. used stacked records from a variety of sources to create a global graph of temperature change. This famous ‘hockey stick’ graph (Figure 4) clearly demonstrates the rapid rate of change since the industrial revolution, with temperature rise sharply accelerating over the last 150 years.
However, a new paper out in Science by Marcott et al. (2013) extends Figures 3 and 4 back to the last 11,300 years, with the Earth’s temperatures now warmer than the Earth was 4000 years ago (Figure 5). During the last 200 years, temperatures have rocketed up, confirming earlier reconstructions by Mann et al. 2008.
Although the Earth has not yet exceeded the temperatures of the Early Holocene (5000 to 11,000 years ago), global temperatures have risen from cooler than 95% of the Holocene at around 1900 to warmer than 72% of the Holocene in the last 100 years (Figure 5). This means that, in the last 100 years, the Earth’s temperature has reversed a long-term cooling trend that began around 5000 years ago to become near the warmest temperatures during the last 11,000 years. Furthermore, climate models predict that the Earth’s temperature will exceed the warmest temperatures of the Holocene by 2100, regardless of which greenhouse gas emission scenario is used .
However, Marcott et al. do note in their paper that the ‘uptick’ shown in the graph is not statistically robust, as the median resolution of all data is 120 years. However, this has been shown by other authors, such as Anderson et al. 2013, who demonstrate rapid warming over the last century from geological data.
However, it doesn’t end there. Using more proxies to extend the temperature graph back to the last glacial and modelling scenarios from the IPCC for the next 100 years, the graph below (Figure 6) has been created. Dubbed “The Wheelchair”, it shows the current and future alarming rate of temperature increase, compared with temperature fluctuations over the last 20,000 years. This figure was created by Jos Hagelaars.
The role of carbon dioxide
Carbon dioxide is a key factor in climate change. Carbon dioxide concentrations fluctuate annually, as seen in Figure 7 from the longest direct measurements of CO2 from Mauna Loa in Hawaii. The seasonal fluctuation is caused by variations in uptake of carbon dioxide by land plants. However, looking at Figure 2 above, you can see that this is just part of a much longer record of carbon dioxide. The IPCC 2007 Synthesis Report stated:
“Global GHG emissions due to human activities have grown since pre-industrial times, with an increase of 70% between 1970 and 2004”.
These increases in carbon dioxide are primarily due to increased use of fossil fuels, but land use changes also provide a significant contribution.
Because humans are constantly adding more carbon dioxide to the atmosphere, we are changing its balance and affecting the Earth’s climate. The carbon dioxide and other atmospheric gasses in the atmosphere combine with changes in land cover and solar radiation to drive climate change. They affect the absorption, scattering and emission of radiation within the Earth’s atmosphere, resulting in a positive energy balance and warming influences on our climate.
The IPCC 2007 states that there is very high confidence that the world has warmed since 1750, and that the combined radiative forcing due to increases in carbon dioxide, methane and nitrous oxide is 2.3W/m2, and that the rate of increase during the industrial era is was unprecedented in the last 10,000 years.
Changes in solar irradiance since 1750 AD have caused a small radiative forcing of +0.12 W/m2.
But how has carbon dioxide changed over the last few thousand years?
Figure 8 shows the long-term temperature and carbon dioxide record from Antarctica. Carbon Dioxide (blue line) now approaches 400 parts per million, which is far higher than it has been at any point in the last 400,000 years. Methane (green line) is also higher than it has been before over this timescale. You can also see how closely temperature (red line) has tracked carbon dioxide and methane over this time. The temperature in Antarctica (red line) is expected to continue to rise with this increase in atmospheric greenhouse gases.
Climate change in Antarctica
The western Antarctic Peninsula warmed by 2.5°C from 1950-2000 (Figure 9). The mean annual temperature is rising, with the -9°C isotherm moving southwards, resulting in the collapse of several ice shelves (28,000 kmof ice shelf has been lost since the 1990s). Climate records from the South Orkney islands suggest that warming began in earnest in the 1930s.
This warming has been related to variations in the belts of upper atmosphere winds that encircle the Antarctic continent (the Circumpolar Vortex). Periodic oscillations in this result in the periodic strengthening and weakening of this belt of winds. A strengthening of this vortex has been associated with changes in surface atmospheric pressures. This pressure pattern causes northerly flow anomalies, which in turn cause cooling over East Antarctica and warming over the Antarctic Peninsula.
Other factors contributing to the recent regional rapid warming over the Antarctic Peninsula include decreased sea ice in the Bellingshausen Sea, resulting in warmer air temperatures, and decreasing precipitation over the south western peninsula[10, 11]. Finally, since the 1950s, the Antarctic Circumpolar Current has warmed by +0.2°C, with the warming greatest near the surface. Waters west of the Antarctic Peninsula have also warmed rapidly[8, 13].
This video from NASA shows global warming since 1884, and you can really see the large and rapid warming around the Antarctic Peninsula over the last 50 years.
Observed impacts of climate change
The impacts of this recent regional rapid warming around the Antarctic Peninsula have been dramatic, with the collapse of ice shelves, and with 87% of glaciers in recession. The present-day ice loss from the Antarctic Peninsula is -41.5 giga-tonnes per year. Ice-shelf tributary glaciers have become destabilised following ice-shelf collapse[17, 18]. Other glaciers have thinned, accelerated and receded as a result of increased melting (see Recent Change) [19, 20]. The rapid shrinkage of glaciers around the Antarctic Peninsula, coupled with the potential for ice-shelf collapse and grounding line retreat, raises concerns for the future of the West Antarctic Ice Sheet, and this is an area of urgent current research. This is covered in more detail under Marine Ice Sheet Instability.
- Marine Ice Sheet instability
- Ice shelves
- Sea level rise
- Glacier recession in Patagonia
- Glacier recession on the Antarctic Peninsula
- Glaciers and climate change
- Antarctica’s contribution to global sea level rise
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