Climate change, which refers to any long-term trends in climate over many years or decades, around which climate variability may be evident year on year, is one of the greatest socio-economic and ecological challenges facing us today. By its definition, a single warmer or cooler year on its own is not sufficient evidence to assert that climate is changing, but systematic changes in average conditions over many years do provide evidence of a changing climate. Climate change continues to be discussed as the overarching issue this generation and those to follow must address.
The general consensus is that it is undermining development and increasing the burdens on the poorest people in the world, who are often hardest hit by weather catastrophes, desertification, and rising sea levels, among others. In general it is regarded as capable of having serious devastating effects on the three dimensions or pillars of sustainable development – economic, social and environmental. The scientific basis of climate change may be known to experts and professionals, but society is increasingly seeking information about the nature of the evidence and what can be done in response to a changing climate. This article provides some of that much-needed simplified information on the science of climate change.
In its natural state, the Earth’s climate system is a dynamic system, always in transformation from one state to another, sometimes smoothly and sometimes turbulently. Thus, looking back to the Earth’s history, it is not surprising that its climate system has always changed. In general, global temperatures have been cycling or alternating between geological intervals of warmer global average temperature known as Interglacial Periods, and periods of colder global average temperature, known as Glacial Periods. In general, long glacial periods are therefore separated by more temperate but shorter interglacials. These changes are driven by both external influences and dynamics internal to the Earth system. Key external influences include fluctuations in the amount of energy emitted by the Sun, and changes in the Earth’s orbit and axial tilt that affect the intensity and distribution of the Sun’s energy across the Earth. Internal influences on climate include changes in the surface reflectivity due to the presence or absence of ice, changes in atmospheric composition of greenhouse gases, variations in ocean currents, drifting continents, the cooling effect of volcanic dust, and other geological processes.
If climate has always varied in the past, why are we so concerned about the recent warming? The concern is that modern climate is changing far more quickly than in the geological past. In addition, scientific evidences are indicating that recent climate changes cannot be explained by natural causes alone. In general, research indicates that while climate changes prior to the Industrial Revolution in the 1700s can be explained by natural causes, such as changes in solar energy, volcanic eruptions, and natural changes in atmospheric gas concentrations, natural causes are very unlikely to explain most observed warming, especially warming since the mid-20th century. There is no record of temperatures within human history ever having increased as rapidly as they have over the past 100 years. Rather, human activities can very likely explain most of that warming, especially because of the way we have been affecting and changing the amount of the gases in the earth’s atmosphere (a thin layer of air that envelops the Earth).
The Sun, which powers the Earth’s climate system, radiates energy at very short wavelengths. About 33% the solar energy that reaches the top of earth’s atmosphere is reflected directly back to space. The remaining 67% is absorbed by the surface and, to a lesser extent, by the atmosphere. To balance the absorbed incoming energy, the earth must, on average, radiate the same amount of energy back to space. Because the Earth is much colder than the Sun, it radiates at much longer wavelengths (thermal radiation). Much of this thermal radiation emitted by the land and ocean is absorbed by the atmosphere, including clouds, and reradiated back to earth.
Gases in the atmosphere, such as carbon dioxide (CO2), methane, nitrous oxide and water vapour, are essential to retain heat and keep the planet warm enough to sustain life – hence the reference to them as greenhouse gases. They absorb the radiation released by the Earth’s surface and then radiate heat in all directions, including back towards the ground – adding to the heat the ground receives from the Sun. This is the so called greenhouse effect, which is analogous to the glass walls in a greenhouse which reduce airflow and increase the temperature of the air inside. The Earth’s greenhouse effect warms the surface of the planet. Without the natural greenhouse effect, the average temperature at earth’s surface would be more than 30oC cooler than it is now, and life as we know it would not be possible. Thus, earth’s natural greenhouse effect makes life as we know it possible.
If the greenhouse effect is essential for our living, so why the current concern about greenhouse effect-induced warming and what makes the climate change we are now experiencing different? There is now strong evidence that recent rapid climate changes are driven largely by a range of human activities. Greenhouse gases are released into the atmosphere by burning fossil fuels, clearing forests, cement manufacture, and by many other industrial and agricultural activities, thereby increasing the amount of radiation trapped near the Earth’s surface and driving accelerated warming. Because of this, the atmospheric concentrations of carbon dioxide and other greenhouse gases are higher today than they have been over the last half-million years or longer. Thus, human activities have greatly intensified the natural greenhouse effect, causing global warming. This process, called the enhanced greenhouse effect, is caused by a forced release of greenhouse gases from their terrestrial store into the atmosphere that has no precedent in history. The net effect could throw the climate system into a tumultuous state of extreme weather and climatic conditions.
To further understand the situation, we can look at it with a simple analogy, considering salt and human health. A small amount of salt is essential for human life, and slightly more salt in our diet often makes food tastier. However, too much salt can be harmful to our health. In a similar way, greenhouse gases are essential for our planet; the planet may be able to deal with slightly increased levels of such gases, but too much will affect the health of the whole planet.
Since the beginning of the industrial era (about 1750), the overall effect of human activities has resulted in the increase of CO2 from 280 parts per million (ppm) to about 390 ppm today, with an annual growth rate of 1.5 to 2.0 ppm. This build-up of GHGs threatens to set the earth inexorably on the path to an unpredictably different climate, particularly in terms of temperature and precipitation changes.
Despite global efforts to reduce greenhouse gas (GHG) atmospheric concentrations that have been largely blamed for global warming induced changes in the climate, the recent Greenhouse Gas Bulletin released by the World Meteorological Organization (WMO) indicated that the atmospheric concentrations of the GHGs are increasing. The concentration of carbon dioxide (CO2) reached 390.9 parts per million in 2011, or 140% of the pre-industrial level. Atmospheric methane also reached a new high of about 1813 parts per billion in 2011, or 259% of the pre-industrial level. Atmospheric nitrous oxide in 2011 was about 324.2 parts per billion, or 120% of the pre-industrial level. Further, radiative forcing by long-lived GHGs increased by 30% from 1990-2011, according to the National Oceanic and Atmospheric Administration’s (NOAA) Annual Greenhouse Gas Index. Carbon dioxide accounts for about 80% of this increase.
Anthropogenic-induced global warming and associated changes in climate are now a reality. The global warming is a reality in the light of current developments of temperatures observed since the 19th century. The observed temperatures show a general upward trend across the globe. The average surface temperature has risen by 0.6OC+ or – 0.2OC since 1860. The observations indicate that the 20th century probably experienced the greatest warming of all ages since 1 000 years in the Northern hemisphere. The decades 1990 and 2000 were the warmest of the 20th century. The years 1998, 2005, 2003 and 2002 were the warmest on record since 1861.
Since 1976, the rise in temperature has been sharp, reaching 0.18OC per decade. The linear trend of warming over the last 50 years, from 1956 to 2005 (0.13OC per decade) is almost twice that of 100 years, from 1906 to 2005 (IPCC, 2007). According to the recent Bulletin of the World Meteorological Organization (WMO), the year 2013 was among the top ten warmest years since modern records began in 1850. It tied with 2007 as the sixth warmest year, with a global land and ocean surface temperature that was 0.50OC (0.90OF) above the 1961–1990 average and 0.03OC (0.05OF) higher than the most recent 2001–2010 decadal average.
Human-amplified global warming and the associated increases in global temperatures are changing fundamental climate processes. Some of those changes may be beneficial in some areas, but it is expected that most will cause more harm than good. Oceans are reportedly becoming more acidic as a result of the carbon dioxide uptake, with serious potential repercussions for coral reefs and the underwater food chain. All these point to the fact that the world may continue to face climate change-induced decrease in food security, less predictable availability of fresh water, and adverse health effects with increasing concentrations of GHGs, as predicted in many scientific models.
The past, present and future climate change scenarios in Nigeria will be discussed in the next article.
By Prof. Emmanuel Oladipo (Climate Change Specialist and Adjunct Professor, Department of Geography, University of Lagos, Nigeria. email@example.com)