Updated: June 3, 2021

Climate change is real and is affecting Maryland now

There is no scientific controversy: 99% of scientists agree that our climate is changing and that global temperatures are increasing as levels of heat-trapping greenhouse gases rise in the atmosphere (CO2, methane, nitrogen dioxide, water vapor).

Global measurements of temperature since weather records began in 1884 show a clear long-term trend toward warming. The animation below shows a clear trend in temperature, even though there can be substantial year-to-year and place-to-place variation in weather. 

ABOVE: Yearly observations of global temperatures since 1884. This animation shows the temperatures across the globe as differences from the average temperature between 1951-1980 (the baseline): blue shows areas cooler than that baseline, while yellow-orange shows areas warmer than the baseline. Although there is considerable variation in temperature from place to place and year to year, the overall trend is clearly toward warming.

How do scientists know that climate is changing?

Many different kinds of evidence consistently point to a warming trend on Earth. The graphic below illustrates some of the indicators measured across the globe over many decades that show that the Earth’s climate is warming (white arrows indicate increasing trends, black arrows indicate decreasing trends). These indicators are extremely consistent:  all of the things that are expected to increase in a warming world are increasing, while things expected to decrease in a warming world are all, in fact, decreasing. If climate change were not real, we would be extremely unlikely to see this level of agreement among so many different pieces of evidence. This is why 99% of scientists publishing papers on climate change agree that it is real.

indicators of a warming world

USGCRP. 2014 Third National Climate Assessment. Appendix 3

Measurements show that temperatures are rising

Recorded observations of global air and ocean temperatures across many years provide some of the most direct evidence of a long-term warming trend, as seen in the plot below of the average global air temperature each year since 1880. The fact that temperatures can vary dramatically year-to-year does not contradict the clear long-term warming trend. Although it is tempting to think that a cooler than average summer or even a very cold winter might mean that global warming has slowed or stopped, such short-term variation in local weather averages out over the long term.

increasing global temperatures

USGCRP. 2013 Third National Climate Assessment.

Don’t be distracted by short term variation!

The important difference between short term variation and a trend can be seen in the figure below. 

climate graph showing long term warming

USGCRP. 2013. Third National Climate Assessment.

Looking at all the data since 1970, you can see that even though there is variation in how much the average temperature changes from year to year, the overall direction of temperature change is increasing. However, looking at just a few years at a time can suggest that temperatures aren’t actually rising (flat red lines). These flat lines suggest that there is no increase in the mean temperature over 10 year intervals, but this leaves the increasing height of the three flat lines unexplained. Only looking at the long term data (from 1970-2010) reveals the real pattern. 

The long-term trend toward increasing temperature is underscored by the observation that every decade since 1950 has been warmer than the preceding one (refer to the graph below). 

global temperature change graph


In general, scientists know that focusing on only a few data points will rarely reveal true patterns.  This is cleverly illustrated in this video about a man walking his dog.

Each year is hotter than the last, but the details are slightly different. Observed temperature records show that 2015 was the hottest year ever recorded (NASA (http://data.giss.nasa.gov/gistemp/maps/). These graphs show the global “temperature anomalies”, that is, the difference in temperature at each location from the average temperature recorded during 1951-1980. During 2014, abnormally cool temperatures were seen in the Midwest and MidAtlantic states, but this was not seen in 2015. In both years, cool ocean temperatures are seen off of Greenland, perhaps reflecting the input of cold meltwater from Greenland’s glaciers into the North Atlantic (blue arrows). The large mass of very warm water in the equatorial Pacific in 2015 is the effect of El Nino (black arrow). Comparison of 2014 and 2015 with the same plot for temperatures in 2000 show how much air and ocean temperatures have increased in just the past 15 years. 

global temperature trends

Global temperatures in 2014 & 2015 compared with 2000. Source, NASA.

Why is air temperature rising? The basic answer to this is simple: the concentration of “greenhouse gases” is increasing within the relatively narrow atmospheric layer. The greenhouse gases include carbon dioxide (CO2), methane (CH3), nitrous dioxide (N2O), and water vapor (H2O). These gases slow the loss of heat from the Earth by scattering heat waves in random directions instead of allowing them to pass cleanly through the atmosphere into space. 

human influence on greenhouse effect

Source: USGCRP. 2014Third National Climate Assessment. Appendix 3

Earth has always had a greenhouse effect because there have always been low levels of greenhouse gases in the atmosphere. In fact, without the greenhouse effect, Earth would rapidly become too cold after dark to sustain life. 

Why do we call this the greenhouse effect? Earth has always had a greenhouse effect because there have always been low levels of greenhouse gases in the atmosphere.  In fact, without the greenhouse effect, Earth would rapidly become too cold after dark to sustain life. 

The recent warming of the atmosphere is not due to increased solar radiation. If we chart temperature changes since 1900 caused only by natural causes like volcanoes and solar activity, the temperature trend (blue line) does not correspond to the observed temperature increase (black line). Only by including temperature changes caused by the release of CO2 into the atmosphere by humans produces results that match the observed data.

human and natural influences on climate

Temperatures are plotted as the difference from the average between 1890-1919 Source: USGCRP. 2014Third National Climate Assessment. Appendix 3

carbon emissions

Temperatures are plotted as the difference from the average between 1890-1919  Source: USGCRP. 2014Third National Climate Assessment. Appendix 3

Burning fossil fuels can account for the increase in CO2.  The amount of coal, oil and natural gas burned since 1850 and the CO2 that burning has emitted is a matter of record, and is shown in Fig. 11. The rapid rise of CO2 attributable to burned fossil fuels closely corresponds to the rise of temperatures in the same period. 

Haven’t temperature and CO2 always gone up and down?  Absolutely.  Scientists have measured the concentration of CO2 from minute amounts of air trapped in Antarctic ice cores dating back 800,000 years.  Other material in the cores allows the temperature at each time to be inferred. The cycles seen in the figure below show that temperature and CO2 concentration are coupled—when CO2 rises, temperature rises. The low temperatures correspond to the Ice Ages.

carbon emissions

Atmospheric CO2 concentrations (top, blue line) and air temperatures (bottom, red line) have increased and decreased together for the past 800,000 years. Note that CO2 concentration had never been over 300 ppm until after the 1900s.  Source: National Research Council 2012

CO2 concentrations have never before been above 300 parts per million (ppm). The problem now is that the concentration of greenhouse gases is both greater than at any time in the past 800,000 years and is rising at an unprecedented rate. In 2013, the Earth exceeded 400 ppm, which is so far out of the normal range that it cannot have been due to the same causes responsible for previous CO2 peaks. 

The astonishing speed at which the concentration of CO2 has increased since the industrial revolution exceeds any previous increase in Earth’s history, and can be seen here. 

Why do we call this climate change and not just global warming?  Rising air temperatures have a myriad of interacting effects, causing strange and violent weather and produce a syndrome of flooding, drought and wildfires. 

global warming components

Most of the increased air temperature from the greenhouse effect has been absorbed by the ocean. Source:  Skepticalscience.com, data from IPCC AR4.

This variety of effects occur because most of the increased air temperature has been absorbed by the ocean. More evaporation occurs from a warming ocean, and warmer air holds more of this water vapor. Major tropical storms derive energy from heat in the ocean, and a warmer ocean increases the energy and severity of storms such as Hurricane Sandy. Storms with extra energy can gather and release more water both because their winds gather water from a larger area and because there is more water vapor in the air.

sea surface temperature

Source: USGCRP. 2014Third National Climate Assessment. Ch. 1

Curiously, the same factors that cause increases in downpours and flooding also cause and increase the severity of drought   In the Northeast, increased rain is predicted for winter, but summers are likely to be dry.  Because warmer air and soil temperatures cause water in the soil to evaporate more quickly droughts in our region are likely to become deeper. 

By Dr. Sara Via, Depts. Biology & Entomology, University of Maryland 

Additional resources

Basic information on climate change

National Research Council. 2012.  Climate Change:  Evidence, Impacts and Choices.  Answers to common questions about climate change.  https://nas-sites.org/americasclimatechoices/more-resources-on-climate-change/climate-change-lines-of-evidence-booklet/

National Research Council. 2012.  Climate Change:  Evidence, Impacts and Choices.   Seven short videos covering the basics of climate change. https://www.youtube.com/playlist?list=PL38EB9C0BC54A9EE2

J. M. Melillo, Terese (T.C.) Richmond, and G. W. Yohe, Eds., U.S. Global Change Research Program. 2014.  Climate Change Impacts in the United States: The Third National Climate Assessment, 790-820. doi:10.7930/J0G15XS3. http://nca2014.globalchange.gov/

How is Climate Change Affecting your County? | Cornell University

What Will Climate Feel Like in 60 Years? | UM Center for Environmental Science

Resources on sea level rise

Chesapeake Quarterly and Bay Journal.  2014.  Come High Water.  Articles and videos about impacts of rising sea level on Chesapeake Bay communities. http://www.chesapeakequarterly.net/sealevel/index.php#Articles

UMCES Ian Newsletter.  2008. Maryland at risk. Sea-level rise adaptation and response. http://ian.umces.edu/enewsletter/2008/October/

Union of Concerned Scientists. 2017. When rising seas hit home.