Climate Change - Realities, Impacts Over Ice Cap, Sea Level and Risks

Miles of Ice Collapsing Into the Sea

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Prior to the satellite era, solar output was estimated by several methods, including methods based on long-term records of the number of sunspots observed each year, which is an indirect indicator of solar activity. These indirect methods suggest that there was a slight increase in solar energy received by the Earth during the first few decades of the 20th century, which may have contributed to the global temperature increase during that period see Figure 2. Perhaps the most dramatic example of natural climate variability is the ice age cycle.

Detailed analyses of ocean sediments, ice cores, geologic landforms, and other data show that for at least the past , years, and probably the past several million years, the Earth has gone through long periods when temperatures were much colder than today and thick blankets of ice covered much of the Northern Hemisphere including the areas currently occupied by the cities of Chicago, New York, and Seattle.

Through a convergence of theory, observations, and.

As discussed in Appendix D , very unlikely indicates a less than 1 in 10 chance of a statement being incorrect. Detailed worldwide records of fossil fuel consumption indicate that fossil fuel burning currently releases over 30 billion tons of CO 2 into the atmosphere every year Figure 2. Tropical deforestation and other land use changes release an additional 3 to 5 billion tons every year.

Precise measurements of atmospheric composition at many sites around the world indicate that CO 2 levels are increasing, currently at a pace of almost 2 parts per million ppm per year. We know that this increase is largely the result of human activities because the chemical signature of the excess CO 2 in the atmosphere can be linked to the composition of the CO 2 in emissions from fossil fuel burning. Moreover, analyses of bubbles trapped in ice cores from Greenland and Antarctica reveal that atmospheric CO 2 levels have been rising steadily since the start of the Industrial Revolution usually taken as ; see Figure 2.

The current CO 2 level ppm as of the end of is higher than it has been in at least , years. For further details see Figures 6. Based on data from Boden et al.

Only 45 percent of the CO 2 emitted by human activities remains in the atmosphere; the remainder is absorbed by the oceans and land surface. The combined impacts of rising CO 2 levels, temperature change, and other climate changes on natural ecosystems and on agriculture are described later in this chapter and in further detail in Part II of the report. Human activities have led to higher concentrations of a number of GHGs as well as other climate forcing agents. For example, the human-caused increase in CO 2 since the beginning of the Industrial Revolution is associated with a warming effect equivalent to approximately 1.

Although this may seem like a small amount of energy, when multiplied by the surface area of the Earth it is 50 times larger than the total power consumed by all human activities. In addition to CO 2 , the concentrations of methane CH 4 , nitrous oxide N 2 O , ozone O 3 , and over a dozen chlorofluorocarbons and related gases have increased as a result of human activities. Collectively, the total warming associated with GHGs is estimated to be 3. While CO 2 and N 2 O levels continue to rise due mainly to fossil fuel burning and agricultural processes, respectively , concentrations of several of the halogenated gases are now declining as a result of action taken to protect the ozone layer, and the concentration of CH 4 also appears to have leveled off see Chapter 6 for details.

Human activities have also increased the number of aerosols, or particles, in the atmosphere. While the effects of these particles are not as well measured or understood as the effects of GHGs, recent estimates indicate that they produce a net cooling effect that offsets some, but not all, of the warming associated with GHG increases see Figure 2. Averaged over the globe, it is estimated that these land use and land cover changes have increased the amount of sunlight that is reflected back to.

Positive forcing corresponds to a warming effect. See Chapter 6 for further details. Other human activities can influence local and regional climate but have only a minor influence on global climate. The response of the climate system to GHG increases and other climate forcing agents is strongly influenced by the effects of positive and negative feedback processes in the climate system. One example of a positive feedback is the water vapor feedback. Water vapor is the most important GHG in terms of its contribution to the natural green-.

Because the rate of evaporation and the ability of air to hold water vapor both increase as the climate system warms, a small initial warming will increase the amount of water vapor in the air, reinforcing the initial warming—a positive feedback loop.

If, on the other hand, an initial warming were to cause an increase in the amount of low-lying clouds, which tend to cool the Earth by reflecting solar radiation back to space especially when they occur over ocean areas , this would tend to offset some of the initial warming—a negative feedback. Other important feedbacks involve changes in other kinds of clouds, land surface properties, biogeochemical cycles, the vertical profile of temperature in the atmosphere, and the circulation of the atmosphere and oceans—all of which operate on different time scales and interact with one another in addition to responding directly to changes in temperature.

The collective effect of all feedback processes determines the sensitivity of the climate system, or how much the system will warm or cool in response to a certain amount of forcing. A variety of methods have been used to estimate climate sensitivity, which is typically expressed as the temperature change expected if atmospheric CO 2 levels reach twice their preindustrial values and then remain there until the climate system reaches equilibrium, with all other climate forcings neglected. Most of these estimates indicate that the expected warming due to a doubling of CO 2 is between 3.

Unfortunately, the diversity and complexity of processes operating in the climate system means that, even with continued progress in understanding climate feedbacks, the exact sensitivity of the climate system will remain somewhat uncertain.

The Emergency

Nevertheless, estimates of climate sensitivity are a useful metric for evaluating the causes of observed climate change and estimating how much Earth will ultimately warm in response to human activities. Many lines of evidence support the conclusion that most of the observed warming since the start of the 20th century, and especially over the last several decades, can be attributed to human activities, including the following:. The vertical pattern of observed warming—with warming in the bottom-most layer of the atmosphere and cooling immediately above—is consistent with warming caused by GHG increases and inconsistent with other possible causes see below.

Detailed simulations with state-of-the-art computer-based models of the climate system are only able to reproduce the observed warming trend and patterns when human-induced GHG emissions are included. In addition, other possible causes of the observed warming have been rigorously evaluated:. As described above, the climate system varies naturally on a wide range of time scales, but a rigorous statistical evaluation of observed climate trends, supported by analyses with climate models, indicates that the observed warming, especially the warming since the late s, cannot be attributed to natural variations.

Satellite measurements conclusively show that solar output has not increased over the past 30 years, so an increase in energy from the Sun cannot be responsible for recent warming. There is evidence that some of the warming observed during the first few decades of the 20th century may have been caused by a slight uptick in solar output, although this conclusion is much less certain. Direct measurements likewise show that the number of cosmic rays, which some scientists have posited might influence cloud formation and hence climate, have neither declined nor increased during the last 30 years.

Moreover, a plausible mechanism by which cosmic rays might influence climate has not been demonstrated. Based on these and other lines of evidence, the Panel on Advancing the Science of Climate Change—along with an overwhelming majority of climate scientists Rosenberg et al. In order to project future changes in the climate system, scientists must first estimate how GHG emissions and other climate forcings such as aerosols and land use will change over time. Since the future cannot be known with certainty, a large number.

These scenarios have increased in complexity over time, and the most recent scenario development efforts include sophisticated models of energy production and use, economic activity, and the possible influence of different climate policy actions on future emissions. Future climate change, like past climate change, is also subject to natural climate variations that modulate the expected warming trend.

After future forcing scenarios are developed, climate models are used to simulate how these changes in GHG emissions and other climate forcing agents will translate into changes in the climate system. Climate models are computer-based representations of the atmosphere, oceans, cryosphere, land surface, and other components of the climate system. All climate models are fundamentally based on the laws of physics and chemistry that govern the motion and composition of the atmosphere and oceans.

The most sophisticated versions of these models—referred to as Earth system models —include representations of a wide range of additional physical, chemical, and biological processes such as atmospheric chemistry and ecosystems on land and in the oceans.

Climate Change

The resolution of climate models has also steadily increased, although global models are still not able to resolve features as small as individual clouds, so these small-scale processes must be approximated in global models. After decades of development by research teams in the United States and around the world, and careful testing against observations of climate over the past century and further into the past, scientists are confident that climate models are able to capture many important aspects of the climate system.

Scientists are also confident that climate models give a reasonable projection of future changes in climate that can be expected based on a particular scenario of future GHG emissions, at least at large continental to global scales. A variety of downscaling techniques have been developed to project future climate changes at regional and local scales.

These techniques are not as well established and tested as global climate models, and their results reflect uncertainties in both the underlying global projections and regional climate processes. Hence, predictions of regional and local climate change are generally much more uncertain than large-scale changes. The most recent comprehensive modeling effort to date included more than 20 different state-of-the-art climate models from around the world.

Each of these climate models projected future climate change based on a range of different scenarios of future GHG emissions and other changes in climate forcing. Continued warming is projected by all models, but the trajectory and total amount of warming varies from model to model and between different scenarios of future climate forcing.

Based on these results, the IPCC estimates that global average surface temperatures will rise an additional 2. The wide spread in these numbers comes from uncertainty not only in exactly how much the climate system will warm in response to continued GHG emissions, but also uncertainty in how future GHG emissions will evolve. As with observed climate change to date, there are wide geographic variations in the magnitude of future warming, with much stronger projected warming over high latitudes and over land areas see Figure 2.

In the United States, temperatures are projected to warm substantially over the 21st century under all projections of future climate change USGCRP, a. Temperature increases over the next few decades primarily reflect past emissions and are thus similar across different scenarios of future GHG emissions. However, by midcentury and especially at the end of the century, higher emissions scenarios e.

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In addition to increasing global average temperatures, a host of other climate variables are projected to experience significant changes over the 21st century, just as they have during the past century. For example, it is very likely 8 that. Heat waves will become more intense, more frequent, and longer lasting, while the frequency of cold extremes will continue to decrease;. As discussed in Chapter 6 , none of the scenarios considered in this modeling effort attempted to represent how climate policy interventions might influence future GHG emissions.

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