Cambio Climático IPCC Actualización 2013

2. What have been the observed climate changes in the last centuries?

• 2.1 How has the global mean temperature on earth evolved?
• 2.2 How have precipitations and droughts globally changed in the last decades?
• 2.3 Has the salinity, acidity and oxygen content of oceans changed?
• 2.4 How has the earth’s ice cover evolved in the last decades?
• 2.5 Has the level of the oceans changed?
• 2.6 Have extreme weather events increased in the last decades?
• 2.7 How have emissions of greenhouse gases evolved in the last centuries?

2.1 How has the global mean temperature on earth evolved?

It is certain that the global mean surface temperature of the earth has increased since the beginning of the instrumental record. This warming has been about 0.85°C from 1880 to 2012 with an increase of about 0.72°C from 1951 to 2012.. More specifically, each of the last three decades has successively been the warmest on record. They also have very likely been the warmest in the last 800 years and likely the warmest in the last 1400 years.

Several independent data records are in agreement that both land and sea surface temperatures have increased. There is a substantial variability between decades in the rate of warming, and the rate of warming over the last 15 years is smaller than the rate since the 1950s.

Climate reconstructions show warm periods during the middle ages (950 to 1250), and there is high confidence that temperatures during those periods were as warm as the late 20^th century in some regions. However, these were regional warm periods and were not as synchronous across regions as the current warming.

Cities and land use change can contribute to local increases in temperatures, but when the global average is considered, it is likely that these changes have contributed by more than 10% of the reported trend.

In the higher atmosphere, it is virtually certain that since the middle of the 20^th century the troposphere (the lowest 10-15 km of the atmosphere) has warmed and that the stratosphere (the layer between 10-15km and 50-60 km of altitude) has cooled. This is one of the observations that suggests that heat is being trapped in the lower atmosphere by the greenhouse effect. There is no clear agreement however on the rate at which this change is taking place.

Warming of the ocean has also been observed over the course of the 20^th century. It is virtually certain that the upper ocean (above 700 m) has warmed, likely that the ocean warmed between 700 and 2000 m, likely that the ocean warmed from 3000 m to the bottom. However, no significant trend was observed between 2000 m and 3000 m.

Since at least circa 1970, the planet earth has been in energy imbalance, with more energy from the sun entering than exiting the top of the atmosphere and the warming of the oceans accounts for most (93%) of that increase in energy capture.

The large variability in atmospheric circulation between years and between decades makes conclusions difficult to reach about the changes in phenomena such as the North Atlantic Oscillation and the El Nino Southern Oscillation. More...

2.2 How have precipitations and droughts globally changed in the last decades?

On the global scale, it is not clear if there were changes in precipitation and cloud cover, in part because there is insufficient data. The humidity of the lower atmosphere has very likely increased since the 1970s, but it is not clear what changes in precipitations this has led to.

The most recent and most comprehensive analyses do not support anymore that global runoff has increased during the 20^th century as it was concluded in the 4^th assessment report of 2007 (AR4). New results also indicate that global increasing trends in droughts since the 1970s are no longer supported, as it was assumed in the 2007 AR4 conclusions. More...

2.3 Has the salinity, acidity and oxygen content of oceans changed?

Changes have been observed in ocean properties during the past forty years, including temperature, salinity, sea level, carbon content, pH and oxygen level. The observed patterns of change are consistent with changes in the surface ocean (warming, changes in salinity, and an increase in carbon content) in response to climate change and variability and with known physical and biogeochemical processes in the ocean, providing high confidence in this assessment.

The salinity of the surface of the oceans is an important driver of ocean circulation, and it directly related to the balance between evaporation and precipitation. It is very likely that regional trends have enhanced the contrasts between the surface waters in the evaporation-dominated mid-latitudes that are becoming more saline, and surface waters in rainfall-dominated tropical and polar regions that are relatively more fresh. It is very likely that observed changes in surface and subsurface salinity are due in part to human influence.

It is very likely that the uptake of CO₂ by the oceans results in a gradual acidification of the ocean. The pH of seawater has decreased by 0.1 since the beginning of the industrial era, corresponding to a 26% increase in hydrogen ion concentration. Results from laboratory, field, and modeling studies, as well as evidence from the geological record, clearly indicate that marine ecosystems are highly susceptible to the increases in oceanic CO₂ and to the corresponding decreases in pH and carbonate ion.

The warming of the open ocean leads to a reduction in oxygen concentration, since warmer waters can hold less dissolved oxygen (solubility effect), and that warming-induced stratification leads to a decrease in the transport of dissolved oxygen from surface to subsurface waters (stratification effect). More...

2.4 How has the earth’s ice cover evolved in the last decades?

There is very high confidence that the Arctic sea ice extent (annual, multiyear and perennial) has decreased over the period 1979–2012. The rate of the annual decrease was very likely and raised between 3.5 and 4.1% per decade. There is medium confidence that the most recent (1980 to 2012) Arctic summer sea-ice decrease is unprecedented in the last 1,450 years.

By contrast, it is very likely that the annual Antarctic sea ice extent increased at a rate of between 1.2 and 1.8% per decade between 1979 and 2012, due to a decrease in the percentage of open water within the ice pack. There is high confidence that there are strong regional differences, with some Antarctic regions increasing in ice extent/area and some decreasing. There is high confidence that parts of Antarctica floating ice shelves are indeed undergoing substantial changes.

There is very high confidence that —with a very few regional exceptions—terrestrial glaciers world-wide have shrunk in the last decades. There is high confidence that current glacier extents are out of balance with current climatic conditions, indicating that glaciers will continue to shrink in the future, even without further temperature increase.

More specifically, there is very high confidence that the Greenland Ice Sheet has lost ice during the last two decades. There is high confidence that the Antarctic Ice Sheet has also been losing ice during the last two decades.

In the Northern Hemisphere there is also very high confidence that snow cover extent has decreased, especially in spring. In the Southern Hemisphere, evidence is too limited to conclude whether changes have occurred.

Permafrost¹ temperatures have increased in most regions around the world since the early 1980s (high confidence). There is a high confidence that these increases were in response to increased air temperature, and changes in the timing and thickness of snow cover. More...

¹ Permanently frozen subsoil, occurring throughout the Polar Regions and locally in perennially frigid areas.

2.5 Has the level of the oceans changed?

There are two primary contributions to changes in the level of oceans :

1. the expansion of the ocean water as it warms;
2. the transfer to the ocean of water currently stored on land, particularly from glaciers and ice sheets.

During the warm intervals of the middle Pliocene (3.3 to 3.0 million years ago), when there is medium confidence that global mean surface temperatures were 2°C to 3.5°C warmer than at the late pre-industrial period, the evidence suggests that global mean sea level was above present levels but there is high confidence that it did not exceed 20 m above present levels. It seems that at that period the Antarctic Ice sheet had significantly thawed, and that Greenland had essentially melted completely.

There is very high confidence that maximum global mean sea level during the last interglacial period (129,000 to 116,000 years ago) was, for several thousand years, at least 5 m higher than present and there is high confidence that it did not exceed 10 m above present. There is also high confidence that this change in sea level occurred in the context of surface temperatures being at least 2°C warmer than present. After the Last Glacial Maximum, several thousand years ago, the global mean sea levels reached close to present-day values.

There is medium confidence that the magnitude of sea level variations did not exceed 0.25 m per century over the past few millennia. In this context, the current rate of global mean sea level rise is, with medium confidence, unusually high in the context of the last two millennia.

it is virtually certain that the rate of global mean sea level rise has accelerated from relatively low rates in the order of tenths of mm per year in the past millennia to the present rates in the order of mm per year. More specifically, the global mean sea level has risen by 0.19 [0.17 to 0.21] m over the period 1901– 2010. More...

2.6 Have extreme weather events increased in the last decades?

For the purpose of this evaluation, ‘extreme event’ is defined as something that diverges from the normal and are thus rare events. Recent analyses of extreme weather events between 1951 and 2010 generally support the conclusions of previous reports that it is very likely that the number of cold days and nights has decreased and the number of warm days and nights has increased on the global scale. Globally, there is also medium confidence that the length and frequency of warm spells, including heat waves, has increased since the middle of the 20^th century.

It is likely that since 1950 the number of heavy precipitation events over land has increased in more regions than it has decreased. New results also indicate that global increasing trends in droughts since the 1970s are no longer supported, as it was assumed in the 2007 AR4 conclusions. There is high confidence for the occurrence of droughts of greater magnitude and longer duration during the last millennium than observed since 1900 in many regions. There is also medium confidence that during the Little Ice Age (1450–1850) compared to the Medieval Climate Anomaly (950–1250), more mega-droughts occurred in monsoon Asia and wetter conditions prevailed in arid Central Asia and the South American monsoon region.

Changes in droughts show however important regional changes: since 1950, the frequency and intensity of drought has likely increased in the Mediterranean and West Africa and likely decreased in Central North America and North-West Australia.

it is virtually certain that the frequency and intensity of storms in the North Atlantic has increased since the 1970s although the reasons for this increase are debated. Elsewhere, trends in storminess are more difficult to establish.

With high confidence, during the past five centuries floods larger than those recorded since the 20^th century occurred in Northern and Central Europe, the Western Mediterranean region, and Eastern Asia. By contrast, in the Near East, India, central North America, There is medium confidence that contemporary large floods are comparable or surpass historical floods in magnitude and/or frequency. More...

2.7 How have emissions of greenhouse gases evolved in the last centuries?

In 2011, the atmospheric concentrations of the greenhouse gases carbon dioxide (CO₂), methane (CH₄), and nitrous oxide (N₂O), exceed the range of concentrations recorded in ice cores. The past changes in atmospheric greenhouse-gas concentrations are determined with very high confidence from these polar ice cores which extend now all the way to 800 000 years ago.

With very high confidence, the current rates of rise of greenhouse gases in the atmosphere and the associated increases in the greenhouse gas effect are unprecedented with respect to the more precise records of the last 22,000 years and, with medium confidence, this it is also unprecedented in the past 800,000 years.

In several periods characterized by high atmospheric CO₂ concentrations, there is medium confidence that the global mean temperature of the earth was significantly above pre-industrial levels.

• During the mid-Pliocene (3.3 to 3.0 million years ago), an atmospheric CO₂ concentration between 350 ppm and 450 ppm (medium confidence) occurred while, with medium confidence, the global mean surface temperature was approximately 2°C to 3.5°C warmer () than for pre-industrial period.
• During the Early Eocene (52 to 48 million years ago), atmospheric CO₂ concentration exceeded ~1000 ppm while the global mean earth surface temperature was 9°C to 14°C higher (medium confidence) than in pre-industrial period.

Between 1750 and 2011, CO₂ emissions to the atmosphere from fossil fuel combustion and cement production are estimated to have reached 365 [335 to 395] billion tons (Pg) of carbon. While the emissions from fossil fuels are growing faster and faster, another major contributor to atmospheric carbon emissions is associated with land use change, mainly deforestation. Between 1750 and 2011 it is estimated that this activity has resulted in the release of 180 [100 to 260] billion tons (Pg) of Carbon.

Of this carbon released to the atmosphere from fossil fuel and land use emissions from 1750 to 2011, a little less that half has accumulated in the atmosphere, causing the observed increase of atmospheric CO₂ concentration from 278 ppm [275 to 281] around the year 1750 to 390 ppm in 2011. The remaining amount of carbon released by fossil fuel and land-use emissions has been absorbed by the ocean or reabsorbed by terrestrial ecosystems. The oceans absorb a large portion of the CO₂, and this has very likely resulted in a gradual acidification of the ocean .

The concentration of methane (CH₄) in the atmosphere has increased by a factor of 2.5 since preindustrial period, from about 720 ppb in 1750 to about 1803 ppb in 2011 (Figure TS.5). There is very high confidence that the atmospheric CH₄ increase during the industrial period is caused by anthropogenic activities such as agriculture and emissions from landfill and waste.

Since preindustrial times, the concentration of nitrogen oxides (N₂O) in the atmosphere has increased by a factor of 1.2. Changes in the nitrogen cycle (linked a.o. to nutrients and fertilizers production and use as well as fossil fuel combustion), in addition to interactions with CO₂ sources and sinks, influence the amount of N₂O emissions, both on land and from the ocean. More...