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Elevated atmospheric carbon dioxide concentrations are modifying the natural systems of all continents and oceans thereby affecting life on Earth including humans.


How do we know atmospheric carbon dioxide levels are increasing?

Using air bubbles caught in ice-cores it is possible to determine what carbon dioxide levels have been in the past. Contemporary readings have been taken directly from the atmosphere. An important date to consider when looking at these concentrations is 1750, which is about the time the Industrial Revolution started. Global concentrations of carbon dioxide from about 650,000 years before 1750 up to 1750 ranged between 180 to 300 parts per million (ppm).1 Concentrations rose gradually from 1750 (approximately 278 ppm)until around 1850. After 1850 concentrations rose steadily until around 1960, when they started to rise exponentially. The concentration of atmospheric carbon dioxide was close to 317 ppm in 1958, (a rise of 39 ppm in 208 years) when direct atmospheric carbon dioxide readings were started in Mona Loa, Hawaii. In the last 57 years the concentration level has risen over 80 ppm to the current value of 415.78 ppm in October 2022– higher than it has been for the past 800,000 years.2 (See graph below) Since 2015 the rate of increase has accelerated with 2015 and 2016 having the highest rates on record.35


 Concentrations of atmospheric carbon dioxide over time

Figure 1. Data prior to 1958 were taken from ice-core readings. Data since 1958 were taken from the atmosphere at Mauna Loa in Hawaii, one of the140 sites in 45 countries used by the National Oceanic and Atmospheric Administration, Earth System Research Laboratory, Global Monitoring Division for determining global atmospheric carbon dioxide concentrations.3


How have elevated atmospheric carbon dioxide concentrations modified natural systems?

Elevated atmospheric carbon dioxide concentrations have modified natural systems in two main ways. The first is by the acidification of the ocean and the second is by causing adjustments in the climate.

Ocean acidification

Ocean acidification is caused by carbon dioxide from the atmosphere being absorbed by the ocean. This happens because the concentration of carbon dioxide is lower in the ocean than in the atmosphere.4 Once the carbon dioxide is in the ocean it causes a series of chemical reactions which change the number and types of molecules in the water. These changes cause the water to become more acidic. Continued increases in atmospheric carbon dioxide concentrations will result in higher ocean carbon dioxide concentrations and more acidic water.1,5,6,7

Adjustments in the climate

Energy from the sun passes through the atmosphere to the Earth’s surface (input). Some of the energy is absorbed and some is reflected (output). When carbon dioxide (and other greenhouse gases) are added to the atmosphere it becomes more opaque.8 This means the energy reflected back into space comes from higher in the atmosphere where it is colder, thereby diminishing the total amount of heat leaving the atmosphere. This results in more energy input than output which causes an imbalance in the Earth’s energy budget.8,9  When there is an energy imbalance there are adjustments in atmospheric and surface temperatures in order to bring the energy input and output back into balance.10 These adjustments cause the climate to change. As long as there is more energy coming in than going out, the Earth will continue to become warmer.8

How has the extra heat energy affected natural systems?

Most of the extra heat energy has gone to warm the ocean (especially the shallower depths). The rest has gone to melt ice sheets and glaciers, warm the land, and a very small amount has heated the atmosphere.8,9 But the natural systems of the Earth are not isolated. Instead, they interact with each other - changes in one system can cause changes in others. As such, the effects of the extra heat energy have influenced other systems resulting in sea level rise, extreme weather events, and biodiversity change.

Sea level rise

The two main causes of sea level rise are the expansion of water in the ocean as it becomes warmer and the melting of small glaciers, sea ice, and the West Arctic, Antarctic and Greenland ice sheets.2,11,12

Extreme weather events

Warmer ocean surface temperatures also lead to extreme weather events because greater water evaporation from the ocean increases water vapor content in the atmosphere. This extra moisture then gets added to storm systems - amplifying storm strength.13

Biodiversity change

All of the above changes in natural systems lead to changes in biodiversity because they alter the environmental conditions where animals, plants and other forms of life live. This causes the number and types of species found in a given area to be rearranged. The rearrangement of species found together can affect the interactions between those species and alter ecosystem functions7,14,15,16 upon which humans depend.17


How are humans affected?

Changes in natural systems have detrimental effects on humans including impaired health and earlier death, decreased food and fresh water supplies, property damage and loss, and costs for preventing consequent damages (adaptation). Since all continents and oceans have been affected, every society is vulnerable. The specific vulnerabilities of a given society are influenced by socioeconomic and other conditions and will vary over time and across communities and regions.2,18 Additionally, there is growing evidence that higher carbon dioxide levels will result in some important crops having reduced nutritional content. The amount of protein, iron and zinc found in crops grown in current atmospheric carbon dioxide concentrations was generally higher than the ones grown at the elevated concentrations expected by the end of this century. (wheat: 6.3, 5.1, 9.3%, protein, iron and zinc, respectively; rice: 7.8, 5.2,3.3%; field peas:2.1, 4.1, 6.8%, soybeans: 0.5% increase, 4.1, 5.1; maize (corn): 4.6, 5.8, 5.2%; sorghum: 0, 1.6 increase, 1.3%).37 A similar study comparing the nutritional content of 18 varieties of rice found declines in the amount of protein (10.3%), iron (8.0%), zinc (5.1%) as well as vitamins B1(thiamine, 17.1%), B2 (riboflavin, 16.6%), B5 (pantothenic acid, 12.7%) and B9 (folate, 30.3%), but an increase in vitamin E.38 


What follows is more information about each of the consequences of elevated atmospheric carbon dioxide concentrations mentioned above including observed changes, descriptions of effects on biodiversity and examples of impacts on humans. There is also a brief discussion about ways to deal with too much atmospheric carbon dioxide. For a detailed discussion of ways to mitigate greenhouse gas effects and find out what you can do see the ‘Climate Solutions’ page.


Ocean Acidification

Observed changes

Since the beginning of the industrial era the pH of ocean water has been reduced by 0.1 units, making it more acidic,2 which has become a global problem with regional effects on marine species.7 [Note: The pH scale is logarithmic so for every whole unit decrease in pH there is a 10-fold increase in acidity].5

Biodiversity change

Studies have shown that these chemical changes harm some species and benefit others.1,6 Negative effects include slowed growth and higher rates of mortality which have been recorded in commercially important mollusk species (the Eastern oyster, Atlantic bay scallop and the hard clam).19 Also, certain marine species may experience difficulty forming shells, skeletons and other structures and, in some cases, can actually have their shells dissolve.1,5,6,7 Ocean acidification can also cause shifts in where creatures are found; thus rearranging which combinations of creatures occur together and affecting ecosystems. Pteropods are shell-forming marine species that are a source of food for other creatures. Changes in ocean water chemistry can limit where they are able to live, causing them to be less abundant or absent from some areas, thereby affecting the creatures that depend on them as a food source.20 Also, ocean acidification can slow the rate at which corals produce new structures and increase the rate at which they breakdown. These structures are home to other organisms, so their absence would lead to fewer organisms being able to live in those areas.6



What areas have already been affected?

Areas where conditions are such that shell-forming organisms can be harmed are found in the Pacific, Atlantic, and Indian Oceans. These areas have become larger since preindustrial times.6 Also, these areas are being found at shallower depths than they were in the past. In the North Pacific, these conditions can already be found at very shallow depths and are advancing towards the ocean surface at a rate of 1 to 2 meters (3.28 to 6.6 feet) per year.In the Iceland Sea, the zone that promotes shell formation is becoming shallower at a rate of 4 meters (13 feet) per year.15 Increasingly, there will be more and larger areas that are unsuitable for shell formation and for longer periods of time.7

Human impact examples

How ocean acidification will ultimately affect humans is not known. Using communities such as those in Alaska, where there is a heavy dependence on fishing, as a case study can provide insight into possible impacts. Potential community and state impacts include loss of revenue (commercial fishing $4.6 billion, personal and sports fishing $12.4 billion) and jobs (commercial fishing 90,000, personal and sports fishing 16,000) and for those dependent on subsistence fishing (120,000 people), a loss of important food supplies. Nationally, these commercial harvests help maintain the global balance of trade and provide food for people in the U.S. Pacific Northwest and beyond (data from 2009).7



Land Surface Temperatures

Observed changes

There has been a general warming trend in global mean surface temperature since the late 19th century. The last three decades have been the warmest on record with each decade being warmer than the previous one. How much land surface temperature has changed depends on the years being compared. The temperature average from 2003 to 2012 is about 0.78oC (1.4oF) warmer than that from 1850-1900.2 Warmer land surface temperatures contribute to changes in biodiversity and sea level rise (discussed later).


Biodiversity change

Warmer land surface temperatures have caused changes in biodiversity because species have responded to the new environmental conditions by moving from places that are no longer suitable to ones that are, altering when temperature sensitive activities occur, eventually becoming extinct, or thriving. Each of these responses contributes to the rearrangement of species groups and can affect how ecosystems function. The movement of species to more suitable locations has occurred across the globe and has included species as diverse as birds, plants, algae, insects, and lichens.21 Temperature sensitive activities that happen in spring, such as when plants are in flower or when birds lay their eggs, for different species groups have been estimated to be occurring earlier in the year by an average of 6 days per decade for Birds, 5.5  for Invertebrates, 5 for Amphibians and for plants other than trees, and 3 for Trees.22  Projections of the percentage of species that would become extinct by 2050 based on three scenarios with different levels of temperature increases and atmospheric carbon dioxide concentrations range from 18 to 35%. When projecting percentages to the year 2100 the difference between the scenarios is even greater.14 A number of species, especially pests, seem to be benefiting from adjustments in the climate. Across central Sweden, between the early 1980s and 1994, the incidences of a disease-transmitting tick (Ixodes ricinus) being found on domestic dogs and cats was 22 to 44% greater. During this time there were fewer winter days below -12oC (10.4oF) and more days that were above 10oC (50oF) the rest of the year allowing the ticks to have a longer growing season and greater survival.21


Furthermore, because species interact with each other, the way one species responds to adjustments in the climate will affect other species as well. These interactions are especially important when species are dependent on each other such as a predator or herbivore and their respective sources of food. A drop in ice algae abundances (due to decreases in sea-ice extent and duration that have been occurring since 1976) in the southwest Atlantic (near Antarctica) has negatively affected krill (Euphausia surperba) which has been declining at a rate of 38 to 75% per decade. Krill is a primary source of food for many seabirds, marine mammals and fish.21 Additionally, interactions between species can be disrupted when one species changes the timing of temperature sensitive activities and the other does not. One such example is between caribou and the plants that they eat. In some parts of the Arctic, adjustments in the climate have caused forbs (herbs that are not like grasses) to be most abundant earlier in the year but the caribou that eat them have their calves later in the year - when forbs used to be most abundant.16 In some cases these changed interactions can lead to extinctions. A study that looked at 9,650 systems of species that are dependent upon each other estimated that 6,300 species could become extinct if the species they interact with does.23


Human impact example

Food availability for humans can also be affected by higher land surface temperatures. Crop yields are diminished when there are high daytime temperatures of around 30oC (86oF) during the growing season and generally have been more negatively than positively affected by adjustments in the climate.18


Warmer Ocean Temperatures

Observed changes

There has been approximately a 0.6oC (1.08oF) rise in the average upper layer ocean temperature over the past 100 years.13,15 Three of the consequences of warmer ocean temperatures are changes in biodiversity, extreme weather events and sea level rise (discussed later).

Biodiversity change

Higher ocean temperatures have affected marine species such as plankton, fish, invertebrates, and algae in all ocean basins. These species have moved from low to high latitudes by as much as hundreds of kilometers (1 kilometer = 0.62 miles) per decade because each species is able to live in only a limited range of temperatures. The movement of creatures from the locations where they have lived to places they have not causes changes in which species are found in an area. This, in turn, affects interactions between species and possibly results in less or different food sources being available. Migration patterns, timing of seasonal activities and abundances have also been altered.15,18


Human impact examples

Warmer ocean temperatures can negatively affect people by changing the availability of marine species humans depend on and boosting the presence of pathogens causing human illness and death. Higher water temperatures can be detrimental to commercially important marine species15 and lead to the reorganization of species groups which is expected to have various consequences for fisheries.18 Furthermore, over the past 54 years, as surface water temperatures in the coastal North Atlantic region have increased (up to around 1.5oC, 2.7oF), so have the number of viruses (Vibrio spp.) found in these waters. There has also been an upsurge in the number of humans becoming ill or dying in this region after being exposed to the viruses by swimming, bathing or eating seafood. The problem has escalated in the past 10 years and was at its worst in 2006 during an extreme heat wave in northern Europe.24

Extreme weather events (floods, droughts, heat waves, and wildfires)

Warmer ocean temperatures have caused about a 4% increase in global atmospheric water vapor content since the 1970s. Higher atmospheric water vapor content can contribute to extreme weather events. As ocean surface temperatures increase so does the amount of water that is evaporated from the ocean. The evaporated water goes into the atmosphere increasing water vapor concentrations. This effect is amplified by warmer air temperatures because warm air is able to hold more moisture than cool air. The water vapor is then moved around by atmospheric winds and combined with the moisture in storm systems. This results in more powerful storms (heavier rain or snow) than would have been otherwise - making floods more likely. Heavier storms can enhance the probability of droughts. The total amount of rain that falls does not change much so storms will be fewer and further apart - lengthening dry spells. When a drought lasts longer than a month the soil dries out so there can be no cooling of the surface by evaporation. Without the moderating effect of evaporation surface temperatures become warmer than they would have otherwise. This, in turn, raises air temperatures and dries out plants - the combined effect of which makes wildfires more likely to occur.2,9,13



Human impact examples

Cyclones, floods, heat waves, droughts and wildfires have resulted in human illness and death, damage to infrastructure, interruptions in water supply and food production, changes in ecosystems, and have lead to rising economic losses. A 2009 heat wave in Victoria, Australia caused over 300 deaths and intense brush fires caused 173 deaths and the loss of at least 2,000 buildings. Drought conditions in much of New Zealand from 2007-2009 caused a loss of NZ $3.6 billion. Human health can be adversely affected not only by the direct effects of extreme weather events, alterations in precipitation, and variability in temperatures but also indirectly from shifting patterns of organisms that transmit diseases or crop failures and displacement after a prolonged drought or flooding. Ninety thousand people were displaced in Mozambique during the 2008 Zambezi River flood.18



Sea Level Rise

Observed changes

Sea Level

Global mean sea level is estimated to have risen 0.19 m (0.62 feet) and at a rate of 1.7 mm (0.7 inches) per year from 1901 to 2010. From 1993 to 2010 the rate was faster, around 3.2 mm (0.13 inches) per year.2

Ice Melt

Glacier ice loss - from 1971 to 2009 was around 226 billion tons per year and is accelerating.

 Arctic sea ice loss - from 1979 to 2012 was 0.45 to 0.51 million km2 (0.17 to 0.197 million square miles) per decade. (This rate of loss is based on the average amount of area covered with ice in a given year, which changes throughout the year - is largest in winter and smallest in summer).

Greenland ice sheet loss - from 1992 to 2001 was roughly 34 billion tons per year, but from 2002 to 2011 it was about 215 billion tons per year.2

Global mean sea level is estimated to have risen 0.19 m (0.62 feet) and at a rate of 1.7 mm (0.7 inches) per year from 1901 to 2010. From 1993 to 2010 the rate was faster, around 3.2 mm (0.13 inches) per year.2


What are the consequences of sea level rise?

The consequences of sea level rise include the covering of land surfaces with water (submergence), a greater incidence of flooding of coastal lands, movement of saltwater into surface waters and later into ground water, and the erosion of soft cliffs and beaches; all of which contribute to changes in biodiversity.11

Biodiversity change

Sea level rise is having a negative effect on forests and wetlands found near the coast because of flooding, saltwater intrusion, higher water tables, and poor drainage. These effects, as well as erosion and wetland loss and change, have strong negative effects on biodiversity. The loss and alteration of these areas is of concern because they are unique environments with productive vital ecosystems.11 In Maryland and Louisiana forests close to the coast are dying from salt water exposure and wetlands are being drowned.12 On the islands in the Pacific Ocean and in Southeast Asia there have been shifts in where a number of species are found and some creatures have gone extinct.25


Human impact examples

Sea level rise is of concern to humans because worldwide at least 20 million people live below normal high tide levels and over 200 million live in areas subject to flooding from storm surges. Humans living in coastal areas will have to find ways to cope with the resultant persistent problems, the cost of which will include not just actual damages, but also taking corrective measures to prevent further harm.11 Developing countries that are not able to afford reclamation projects may have to abandon coastal areas and move to higher elevations.26


In some places farmland has been lost because it has become salt flats and tidal marshes.12 In the South Pacific people living on small island nations have had to move because parts of the islands are now under water.26



There has been increased flooding of buildings and roads in low-elevation urban areas and of coastal areas during spring high tides.12 In the United States there are already 90 communities experiencing chronic flooding that affects 10% or more the town or city’s usable land 26 or more times per year. By 2035 that number is predicted to increase to 170 or more36. As sea level rises, flooding can reach higher elevations resulting in seawalls, levees and barriers being overcome more frequently.26 If flooding occurs in conjunction with storms, there can be considerable human mortality as happened in France (Storm Xynthia, 2010), Myanamar (Cyclone Nargis, 2008) and the U.S. (Hurricane Katrina, 2005).11


Saltwater movement

Saltwater encroachment is occurring on some islands, such as the Marshall Islands and Tuvalu,26 and into estuaries in some portions of the Mid-Atlantic region. This jeopardizes sources of freshwater.12


Biodiversity Change


How do changes in biodiversity affect ecosystem function?

Changes in the number and types of species found in a given region can affect ecosystems because each species carries out a necessary function within the ecosystem where it is found. Having a greater variety of species increases the chances that these necessary functions will still be carried out if there are fewer species. This will ensure the continued existence of ecosystems27 and those necessary functions upon which humans depend such as insect pollination, purification of soil and water, recycling of organic wastes, and break down of pollutants.17


Human impact example

The exact impact that changes in biodiversity will have is unknown. As mentioned previously, possible impacts on humans include a reduction in sources of food and revenue, relocation to less impacted areas and disruption of necessary ecological processes. In places where changes result in biodiversity loss there can be an increase in the likelihood of humans contracting diseases such as West Nile virus,28,29,30 Lyme disease31 and Hantavirus.32 Some disease causing pathogens (virus, bacteria, or other microorganism) require hosts (where they live) and a way to transmit the disease from host to host (a vector). Some hosts are better places for the pathogen to live than other hosts. When pathogens are in ‘competent’ hosts their growth and reproduction are promoted and therefore disease transmission is more likely than when the pathogen is in an ‘incompetent’ host where survival and reproduction are not well supported. Areas with numerous different types of species (high species diversity) tend to have both competent and incompetent hosts but areas with a small number of different types of species (low species diversity) tend to have mostly competent hosts. Thus, if humans are in an area with low species diversity the chance that they will be infected by the disease is greater than if they were in an area with high species diversity.32, 33


What can be done about impacts from adjustments in the climate?

Mitigation and adaptation are the two main ways to cope with the negative impacts from modified natural systems. Mitigation strategies include ones that decrease the amount of greenhouse gases in the atmosphere and ones that lessen the amount of solar energy absorbed by the Earth (see ‘Climate Solutions’ page for details). These actions can reduce how much and how fast the climate changes. Adaptation can diminish the amount of damage incurred from impacts that cannot be avoided. For example, adaptation actions for reductions in crop yields from heat and drought are developing and growing stress and heat-tolerant crop varieties or improving irrigation technology. Possible adaptation actions for the reduced growth and survival of commercially valuable shell-forming organisms caused by ocean acidification are protecting areas with naturally low levels of carbon dioxide and catching marine species that are better able to tolerate acidification. Adaptation actions for sea level rise include building sea walls and other coastal protection structures. Used in combination, mitigation and adaptation can improve the ability of humans to withstand the negative impacts of modified natural systems.18

Why is it important to enact mitigation actions as soon as possible?

Mitigation actions can postpone adjustments in the climate thus allowing more time for humans to adapt to a given level of adjustment, perhaps by as much as several decades. If mitigation actions are not taken or are taken later in time, there may be fewer options for withstanding negative impacts. Also, the more extreme changes are and the faster they take place the more likely it is that humans will be incapable of adapting to them. In some parts of the world, the ability to implement development that can withstand future climate adjustment impacts has been reduced because the response to emerging impacts has been inadequate. Furthermore, mitigation actions taken now or in the near future will determine the climate conditions faced by humans around the mid-21st century and beyond.18 Sea level rise and species extinctions exemplify this importance. The effects of sea level rise can be minimized if actions are taken to reduce the amount of atmospheric greenhouse gases as soon as possible because the longer surface waters are heated the greater the amount of water expansion there will be and because thermal expansion is expected to make a greater contribution to sea level rise than ice melting for the rest of this century.2,34 Also, the number of species that will go extinct can be diminished by reducing the amount of greenhouse gasses in the atmosphere and doing so sooner than later.14


How long will it take for mitigation actions to take effect?

Some (approximately 40%) of the effects from greenhouse gases are felt within five years, so changes made now can have an effect in the near future even though a majority of the effects from greenhouse gases will not be felt for a century.8


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Literature cited

Image of earth from, photographs and other images are from

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38  Zhu et al. 2018. Carbon Dioxide (CO2) levels this century will alter the protein, micronutrients, and vitamin content of rice grains with potential health consequences for the poorest rice-dependant countries. Science Advances:4, eaaq 1012.          DOI: 10.1126/sciadv.aaq1012