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Climate change has affected the United States, and the world, in many ways, including threatening its plant life. Global temperatures have increased in the past century, and the top 10 warmest years recorded have all occurred since 1998. The U.S. Environmental Protection Agency has also reported a seven percent increase in greenhouse gas emissions from 1990 to 2014, and net emissions increased by 35 percent from 1990 to 2010 worldwide. Carbon dioxide accounted for three-fourths of all these emissions.

Additionally, there was a 37 percent increase in the warming effect of greenhouse gases from 1990 to 2015, increasing rainfall events in certain areas, droughts in others, raising sea surface temperatures and, among other things, extending crop growing and ragweed pollen seasons. Changes in leaf and bloom dates have been observed as well.

Plants and Climate Change Considerations

Climate change isn’t only about warming. There are various aspects to consider on many fronts, but with plants, here are a few major factors that affect their life cycles.

Minimum, maximum, and average temperatures affect plant growth and distribution. Seasonal cycles are involved as well, but the timing of when regions warm up in the spring and cool down in the fall is changing. For example, warmer weather is setting up later during the spring in many places at high latitudes, where it is staying warmer later into the fall. Arctic areas are warming up the fastest, triggering a change in Arctic tree lines and vegetation growth, which are dependent on summer warmth.

A rising thermometer, however, doesn’t deter all plants. A trend over the last 40 years has been for many North American plant species to move toward warmer areas, and even downhill. University of Washington, Seattle researchers studied 300 plant species in western North America; the 2014 study (published in _Global Change Biolog_y) found that 60 percent of plants studied shifted toward lower elevations as the climate warmed, although the availability of water from increased precipitation is thought to be a driving factor.

Rainfall impacts the balance of plant types in a specific area. Shifts in climate patterns can also alter soil type, affecting which plants thrive and don’t in certain regions. As a result, some species are left behind, particularly ones that have long life cycles and disperse more slowly, such as arctic and alpine plants. The adaptability rates can cause some species to be lost, and others to move. There is also the impact of invasive species, which adapt more quickly to the environmental conditions where native species might struggle.

Temperature, rainfall, and the length of day affect phenophases, or the timing of plant life cycle phases. Seasonal variations impact these phases, but climate change is altering temperature and rainfall patterns, extending growing seasons and shifting them.

Changes in Leaf and Bloom Dates

For some species, such as honeysuckles and lilacs, first leaf and bloom dates vary from year to year. Such variability can make it difficult to measure major changes. The EPA’s Climate Change Indicators show that, since the early 20th century, the growing season in the lower 48 states has increased by about two weeks. More rapid changes have been seen in the West, at a rate of about 2.2 days per decade. In the East, it is about one day per decade. Also, nearly every state has seen the growing season get longer, especially California and Arizona (However, southeastern states such as Alabama and Georgia have seen theirs get shorter). More recent data show earlier final spring frosts and later first fall frosts.

The blooming of plants earlier in the North and West is thought to be connected to this pattern. However, blooms have been occurring later in parts of the South. The pattern is even more pronounced in other places of the Northern Hemisphere.

Scientists are monitoring when first buds appear and when leaves start dropping to measure changes in seasonal patterns. Project BudBurst, implemented by the Chicago Botanical Garden, is facilitating this by enabling the public to report how plants in their city, town, park, or garden respond to the seasons.

Extra Greenery, Climate Change, and CO2

Colder regions have become increasingly more hospitable to plants. In satellite images, a greening effect has been seen across northern landscapes. A concern is that vegetation absorbs sunlight, rather than reflects it like snow and ice do, thereby causing more warming. The thawing of tundra may also release methane, a greenhouse gas. Warmer temperatures are thought to potentially kill off tropical forests, releasing more gases that can contribute to atmospheric warming.

However, a 2016 study in Nature Communications examined a stabilization of atmospheric carbon dioxide increases, which has been attributed to the additional intake o the ground. The belief is that plants being spurred on by climate change is causing it to slow, at least temporarily, because of more carbon dioxide being taken up. The study estimated in the late 20th century, 50 percent of human CO2 emissions were being removed, but up to 60 percent may now be in the process of being absorbed by vegetation. Researchers also found that increased carbon dioxide concentrations help speed up photosynthesis by as much as 40 percent.

Climate Change and Pollen Allergies

Longer plant seasons also equate to more pollen. One example is the ragweed pollen season. Typically peaking in late summer and early fall, ragweed plants can keep churning out pollen until the first frost. Given the current changes, pollen is being produced earlier in the spring and later in the fall. Therefore, pollen counts and allergy seasons are getting longer. According to the EPA, the ragweed season has been extended by five days in Oklahoma since 1995, and by 25 days or more in southern Canada.

Another change is the peak bloom date of cherry blossom trees in Washington, D.C. Since 1921, estimates by the National Park Service show that the peak date has shifted up five days. The Cherry Blossom Festival didn’t even coincide with the peak date in some years. Clearly, climate change is having effects on plants in many ways, including where the grow, fail to thrive, and how human culture interacts with them.