Category Science

          Rising temperatures cause sea levels to rise in two ways. A warmer sea is less dense, so its volume increases and the level rises as it expands. A warmer climate can also cause glaciers to melt into the sea, raising its level.

          The term sea-level rise generally designates the average long-term global rise of the ocean surface measured from the centre of the earth (or more precisely, from the earth reference ellipsoid), as derived from satellite observations. Relative sea-level rise refers to long-term average sea-level rise relative to the local land level, as derived from coastal tide gauges.

          Sea levels are highly variable over periods ranging from seconds to decades. Sea-level rise is the rising trend averaged over longer periods, which is observed at many coastal stations since a few centuries. It is almost certain that global warming due to human emissions of greenhouse gases is responsible for steepening this trend since at least a few decades. The most recent projections for future sea-level rise are presented in the Special IPCC Report on the Ocean and Cryosphere in a Changing Climate (2019). This report is an update of the previous IPCC AR5 report (2013), and includes newer insights in the response of the Greenland and Antarctic ice sheets to global warming. It also provides an estimation of the possible sea-level rise up to the year 2030, see Fig. 1. Two scenarios for greenhouse gas emissions are considered in this figure: (1) a “low” scenario, called RCP2.6, with strong reduction of global greenhouse gas emission, such that global warming will probably not exceed 2 oC; (2) a “high” scenario, called RCP8.5, in which no measures are taken to limit greenhouse gas emissions (‘business as usual’). The high scenario can lead to a rise of up to 5 m of the global average sea level in 2300, but with great uncertainty.

          Several phenomena contribute to sea-level rise. On a global scale, sea-level rise is mainly due to an increase of the water mass and water volume of the oceans. This global sea-level rise (often termed Eustatic sea-level rise) has three components:

(1) thermal expansion of ocean waters related to decrease of the density (also referred to as thermo-steric component of sea-level rise, related to increasing temperature),

(2) water mass increase, which is mainly due to melting of mountain glaciers and decrease of the Greenland and Antarctic ice sheets, and

(3) decreasing storage of surface water and groundwater on land.

Other phenomena can substantially influence sea levels at regional scale, inducing either sea-level rise or sea-level fall. Most important are:

(4) vertical earth crust motions – in particular earth crust adjustment to melting of polar ice caps, the so-called isostatic rebound,

(5) land surface subsidence, related in particular to extraction of groundwater and oil/gas mining and compaction of soft deltaic soils,

(6) changes in the earth gravitational field, related in particular to decrease of the Greenland and Antarctic ice sheets,

(7) regional atmospheric pressure anomalies and changes in the strength and distribution of ocean currents, related in particular to ocean-atmosphere interaction, and

(8) regional sea-level change related to changes in seawater salinity.

          Due to these phenomena, sea-level rise is not uniform around the globe, but differs from place to place. Relative sea-level rise is the locally observed rise of the average sea level with respect to the land level. It is the sum of the components (1-8).

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          Large volcanic eruptions can have an almost immediate effect on the world’s weather. The dust that is thrown into the atmosphere creates a kind of screen, which reflects more of the Sun’s energy back into space. As a result, temperatures around the world can drop slightly and weather patterns may be affected for several years.

          When Mount Pinatubo erupted in the Philippines June 15, 1991, an estimated 20 million tons of sulfur dioxide and ash particles blasted more than 12 miles (20 km) high into the atmosphere. The eruption caused widespread destruction and loss of human life. Gases and solids injected into the stratosphere circled the globe for three weeks. Volcanic eruptions of this magnitude can impact global climate, reducing the amount of solar radiation reaching the Earth’s surface, lowering temperatures in the troposphere, and changing atmospheric circulation patterns. The  extent to which this occurs is an ongoing debate.

          Large-scale volcanic activity may last only a few days, but the massive outpouring of gases and ash can influence climate patterns for years. Sulfuric gases convert to sulfate aerosols, sub-micron droplets containing about 75 percent sulfuric acid. Following eruptions, these aerosol particles can linger as long as three to four years in the stratosphere.

         Major eruptions alter the Earth’s radiative balance because volcanic aerosol clouds absorb terrestrial radiation, and scatter a significant amount of the incoming solar radiation, an effect known as “radiative forcing” that can last from two to three years following a volcanic eruption.

          “Volcanic eruptions cause short-term climate changes and contribute to natural climate variability,” says Georgiy Stenchikov, a research professor with the Department of Environmental Sciences at Rutgers University. “Exploring effects of volcanic eruption allows us to better understand important physical mechanisms in the climate system that are initiated by volcanic forcing.”

          By comparing the climate simulations from the Pinatubo eruption, with and without aerosols, the researchers found that the climate model calculated a general cooling of the global troposphere, but yielded a clear winter warming pattern of surface air temperature over Northern Hemisphere continents. The temperature of the tropical lower stratosphere increased by 4 Kelvin (4°C) because of aerosol absorption of terrestrial longwave and solar near-infrared radiation. The model demonstrated that the direct radiative effect of volcanic aerosols causes general stratospheric heating and tropospheric cooling, with a tropospheric warming pattern in the winter.

        “The modeled temperature change is consistent with the temperature anomalies observed after the eruption,” Stenchikov says. “The pattern of winter warming following the volcanic eruption is practically identical to a pattern of winter surface temperature change caused by global warming. It shows that volcanic aerosols force fundamental climate mechanisms that play an important role in the global change process.”

        This temperature pattern is consistent with the existence of a strong phase of the Arctic Oscillation, a natural pattern of circulation in which atmospheric pressure at polar and middle latitudes fluctuates, bringing higher-than-normal pressure over the polar region and lower-than-normal pressure at about 45 degrees north latitude. It is forced by the aerosol radiative effect, and circulation in winter is stronger than the aerosol radiative cooling that dominates in summer.

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          Fossils contained in layers of rock can reveal details about the climate millions of years ago. Rock that contains a large variety of fossils was formed during a time when the climate was warm; fewer fossils indicate a cooler climate. Rocks that show signs of glacial erosion were part of the Earth’s surface during an Ice Age. Geologists can work out the age of the layers, which tells us when the changes took place.

            Many of the techniques used for palaeoclimatic reconstruction discussed in the preceding sections have only a limited time scale open to their period of study. Most ice cores are restricted to the last million years, whilst tree ring analysis can only provide proxy climate information for at best the last 10,000 years. Ocean sediments provide some of the longest proxy records available, and offer a window on palaeoclimates dating back to the age of the dinosaurs, 100 million years ago. Most older sediments, however, will have been subducted beneath overriding tectonic plates as the continents continue to drift about the Earth. To reconstruct climates older than this, therefore, one needs to look elsewhere for the evidence.

           Sediments laid down on the ocean floor become progressively buried by subsequent debris transported from continental interiors. Deeply buried sediments are subjected to considerable pressures from the overlying layers, and after tens to hundreds of millions of years, the sediments are gradually lithified, forming sedimentary rocks. If, through tectonic movements, these sedimentary rocks are uplifted and exposed, scientists may study them, as they do other forms of evidence, to reconstruct past climates.

          Numerous techniques of analysing sedimentary rocks are used for palaeoclimate reconstruction. Principally, rock type provides valuable insights into past climates, for rock composition reveals evidence of the climate at the time of sediment deposition. However, depositional climatic regimes vary not only due to actual climatic changes but also due to continental movements. The Carboniferous limestones and coals (evidence of warm, humid climates) of Northern England (300Ma), for example, were laid down at a time when Britain was located near the equator, whilst large scale glaciation was occurring in the high latitudes of the Southern Hemisphere.

        The study of rock type is geologically known as facies analysis. Facies analysis investigates how the rock type changes over time, and therefore provides a potential tool for investigating past climatic change. A sedimentary formation consisting of a shale layer (fine-grained mudstone) interbedded between two sandstone layers (coarse-grained), for example, provides evidence of a changing sea level, potentially linked to climatic change (caused either by epeirogeny or ice formation). Sandstones are deposited in coastal zones where the water is shallow, whilst mudstones (shales) are deposited in deeper water of the continental shelf region. A change in the rock type in the vertical cross section must therefore reflect a change in sea level and associated coastline movements.

          Other principal marker rock types include evaporites (lithified salt deposits and evidence of dry arid climates), coals (lithified organic matter and evidence of warm, humid climates), phosphates and cherts (lithified siliceous and phosphate material and evidence of ocean upwelling due to active surface trade winds) and reef limestone (lithified coral reef and evidence of warm surface ocean conditions).

         As well as facies analysis, other techniques, including analysis of sedimentation rates, sediment grain morphology and chemical composition provide information on the climatic conditions prevailing at the time of parent rock weathering. In addition, some of the methods used to reconstruct past climate discussed in earlier sections may be equally applied to sedimentary rocks. For example, the type and distribution of marine and continental fossils within fossil-bearing rocks (principally limestones and mudstones, but occasionally sandstones) are valuable palaeoclimate indicators. Microfossil type, abundance and morphology may also be studied, and palaeotemperatures derived from their oxygen isotope analysis.

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          By studying the growth rings in ancient trees, scientists can gather information about climates of the past. This science is called dendroclimatology. In each year of a tree’s growth, new layers are added to the centre of its trunk, producing a growth ring. Warm, wet growing seasons produce several layers, creating a wide growth ring. In a cold, dry period, fewer layers are produced, and the ring will be narrower.

          The characteristics of the rings inside a tree can tell scientists how old a tree is and what the weather conditions were like during each year of that tree’s life. Very old trees can offer clues about what the climate in an area was like long before measurements were recorded.

         But to understand what the trees tell us, we first have to understand the difference between weather and climate.

          Weather is a specific event—like a rain storm or hot day—that happens over a short period of time. Weather can be tracked within hours or days. Climate is the average weather conditions in a place over a long period of time (30 years or more).

          Scientists at the National Weather Service have been keeping track of weather in the United States since 1891. But trees can keep a much longer record of Earth’s climate. In fact, trees can live for hundreds—and sometimes even thousands—of years!

         One way that scientists use trees to learn about past climate is by studying a tree’s rings. If you’ve ever seen a tree stump, you probably noticed that the top of the stump had a series of rings. It looks a bit like a bullseye.

          These rings can tell us how old the tree is, and what the weather was like during each year of the tree’s life. The light-colored rings represent wood that grew in the spring and early summer, while the dark rings represent wood that grew in the late summer and fall. One light ring plus one dark ring equals one year of the tree’s life.

        Because trees are sensitive to local climate conditions, such as rain and temperature, they give scientists some information about that area’s local climate in the past. For example, tree rings usually grow wider in warm, wet years and they are thinner in years when it is cold and dry. If the tree has experienced stressful conditions, such as a drought, the tree might hardly grow at all in those years.

          Scientists can compare modern trees with local measurements of temperature and precipitation from the nearest weather station. However, very old trees can offer clues about what the climate was like long before measurements were recorded.

          In most places, daily weather records have only been kept for the past 100 to 150 years. So, to learn about the climate hundreds to thousands of years ago, scientists need to use other sources, such as trees, corals, and ice cores (layers of ice drilled out of a glacier).

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            The causes of an Ice Age are not clear. One theory is that the Earth’s tilt and its orbit of the Sun have changed. An orbit that took our planet further from the Sun would result in a cooler climate.

          An ice age is a time where a significant amount of the Earth’s water is locked up on land in continental glaciers.

          During the last ice age, which finished about 12,000 years ago, enormous ice masses covered huge swathes of land now inhabited by millions of people.

          Canada and the northern USA were completely covered in ice, as was the whole of northern Europe and northern Asia.

          At the moment the Earth is in an interglacial period – a short warmer period between glacial (or ice age) periods.

          The Earth has been alternating between long ice ages and shorter interglacial periods for around 2.6 million years.

          For the last million years or so these have been happening roughly every 100,000 years – around 90,000 years of ice age followed by a roughly 10,000 year interglacial warm period.

Causes:

Ice ages don’t just come out of nowhere – it takes thousands of years for an ice age to begin.

          An ice age is triggered when summer temperatures in the northern hemisphere fail to rise above freezing for years. This means that winter snowfall doesn’t melt, but instead builds up, compresses and over time starts to compact, or glaciate, into ice sheets.

          Over thousands of years these ice sheets start to build up – it seems to be in northern Canada when that first happens – and then they spread out across the northern hemisphere.

          “It’s a long term trend over thousands of years to colder summers,” Dr Steven Phipps, an ice sheet modeller, said.

          Dr Phipps is also a climate system modeller and palaeoclimatologist with the University of Tasmania.

          The onset of an ice age is related to the Milankovitch cycles – where regular changes in the Earth’s tilt and orbit combine to affect which areas on Earth get more or less solar radiation.

          When all these factors align so the northern hemisphere gets less solar radiation in summer, an ice age can be started.

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