University of Maryland Extension

Climate Change Basics and Evidence

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.

Diagram showing warming indicatiors
USGCRP. 2014 Third National Climate Assessment. Appendix3

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.

graph showing increasing global land-ocean temperatures

USGCRP. 2013 Third National Climate Assessment. 

figure 6

Fig. 6.  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 Fig. 6.  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 (Fig. 7). 

graph showing increasing temperatures

Fig. 7. Source:  NCDC, NESDIS, NOAA

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 (Fig. 8, NASA ( 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. 

Maps showing global temperatures

Fig. 8.  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. 


Fig. 9.  Source: USGCRP. 2014Third National Climate

Assessment. Appendix3

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?

Greenhouse gases hold heat in the atmosphere much like glass prevents heat loss from a sunny greenhouse (or your car in a sunny parking lot). Because the greenhouse effect causes heat to escape from Earth more slowly than it is added by incoming solar radiation, air temperature in the atmosphere increases over time along with increasing concentrations of greenhouse gases, of which CO2 is found in the largest concentration. 

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.


Fig. 10. Temperatures are plotted as the difference from the average between 1890-1919   Source:  USGCRP. 2014Third National Climate Assessment.  Appendix 3

Graph showing increasing carbon emissions

Fig. 10. 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 Fig. 13 show that temperature and CO2 concentration are coupled—when CO2 rises, temperature rises. The low temperatures correspond to the Ice Ages.


Fig. 12.  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. 


Fig. 13.  Most of the increased air temperature from the greenhouse effect has been absorbed by the ocean.

Source:, data from IPCC AR4.

This variety of effects occur because most of the increased air temperature has been absorbed by the ocean (Figs. 13,14).  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 graph

Fig. 14.   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. 

Effects of climate change are visible in Maryland now! 

Noticeable effects of climate change in Maryland include:

Increasingly Severe Storms

When air temperature increases, more water vapor can be held in the air. Combined with the extra energy available to storms that arise over the warming ocean, the additional water vapor in the air causes more rainfall and increasingly severe winds. The impact of Hurricane Sandy was a tragic illustration of this deadly combination, made even worse by sea level rise along the East Coast. 


In the Northeast (including Maryland), more rain is falling as downpours now than in the past (see fig on right).  This can cause flash flooding and tremendous soil erosion in regions where stormwater is not carefully managed.  In addition, severe inland storms such as the derecho of 2013 are expected to become more common.  Derechos occur when extremely hot air in the southern US collides with colder air in the north.  In 2013, the high winds that resulted from this collision caused wind speeds of 60-80 mph;  that storm moved 600 miles in just 10 hours. 

Sea Level Rise

The MidAtlantic is particularly vulnerable to changing sea level, and is highly likely to see more rise than the global average.  Given Maryland’s miles of coastline and the increased tidal flow in Chesapeake Bay caused by sea level rise, communities on the Eastern Shore and the Bay are extremely vulnerable to flooding.  The nuisance flooding from storms that used to occur periodically in Baltimore and Annapolis is now increasingly frequent and is likely to get worse in coming years.  Between 1957 and 1963, Baltimore had an average of 1.3 floods per year, while Annapolis had 3.8 annually. However, from 2007 to 2013, Baltimore had on average 13.1 flood days per year, while Annapolis had more than 39 such days (more information here—pages from UCS report focusing on Annapolis).  Some Maryland communities are beginning to develop adaptation strategies with help from Maryland’s Department of the Environment (2 refs in Adaptation Basics, in climate change basics Dropbox).  For additional information on how MidAtlantic coastal communities can adapt to sea level rise, see the Climate Resilience Tookit

Storm Surge

Increased sea level makes storm surge during extreme weather on the coast much more serious.  Storm surge causes extensive flooding and damage to properties, and is responsible for many of the injuries that occur during severe coastal storms.  Storm surge impacts not only coastal communities, but also communities on the bay all the way to Baltimore, because increased tidal flux is funneled northward. 

Increasingly Strange Weather

Climate change leads to not only an increase in the average air temperature over time, but is associated with much greater variability in temperature.  During the past few years in Maryland, it has become common to have temperatures fluctuate by 20-30 degrees from day to day.  In Maryland, the extreme weather is not just limited to heat:  In the winters of 2014 and 2015, we had record cold temperatures.  Half of the largest snowstorms on record in the Northeast on have occurred since 2003.  This extreme winter weather has caused some to doubt the reality of global warming, although on a global level, 2014 was the warmest year ever recorded.  Although apparently paradoxical, the extreme winter cold in the Midwest and MidAtlantic is a likely byproduct of climate change, thought to be caused by rapidly warming temperatures in the Arctic that have disturbed circum-polar wind patterns and caused the jet stream to dip southward over North America east of the Rockies.

Basic Information on Climate Change: References and Additional Reading 

National Research Council. 2012.  Climate Change:  Evidence, Impacts and Choices.  Answers to common questions about climate change.

National Research Council. 2012.  Climate Change:  Evidence, Impacts and Choices.   Seven short videos covering the basics of climate change.

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.

 Environmental Protection Agency.  2009.  Frequently asked questions about global warming and climate change:  Back to Basics.  View PDF

Resources on Sea Level Rise

1.  Chesapeake Quarterly and Bay Journal.  2014.  Come High Water.  Articles and videos about impacts of rising sea level on Chesapeake Bay communities.

2.  UMCES Ian Newsletter.  2008.  Maryland at risk.  Sea-level rise adaptation and response.

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

4. Union of Concerned Scientists. 2013.  Causes of sea level rise:  What the science tells us.

Author: Dr. Sara Via, Depts. Biology & Entomology, University of Maryland College Park
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