Happy first day of spring! If you ask a climate scientist, though, the first day of "biological" spring actually arrived two to three weeks ago.

The National Phenology Network's Spring Leaf Index, updated on March 20, shows plants are leafing out up to 20 days earlier than normal along Colorado's urban corridor and eastern plains.

That is also earlier than biological spring in 2012, NPN says. But, parts of coastal Washington and Oregon are two to three weeks late.

The 2017 Spring Leaf Index from the National Phenology Network. March 20, 2017. 

The network had predicted in late February that spring would arrive two to three weeks early across the entire Southeast.

What does that mean for you? Well, if you suffer from seasonal allergies, you're likely already feeling them.

Spring Leaf Index, March 20. (Source: National Phenology Network)

According to the NPN, however, an early spring can bring early-season disease-carriers such as ticks and mosquitos, and have broad-ranging affects on fragile ecosystems and harvesting.

While a longer growing season can result in increased yields for some crops, it is risky because of the higher likelihood of plant damage caused by late frosts or summer drought. Even something as seemingly simple and beautiful as flowers blooming earlier can disrupt the critically important link between wildflowers and the arrival of birds, bees, and butterflies that feed on and pollinate the flowers. Such changes may prove beneficial to some plants and animals, including some harmful invasive ones, but may be detrimental to others. Changes in seasons can affect economically and culturally important outdoor recreation activities, including affecting the timing of hunting and fishing seasons.

The 2017 index is created by researchers and nationwide field observers, who track lilac and honeysuckle blooms, two common and temperature-sensitive flowering plants, a good indicator of biological spring.

Spring Leaf Index: 2017 compared to 2012. (Source: National Phenology Network)

Maps made from this data are then compared to historical maps created in the same way. The data begins in 1981.

Spring Leaf Index as of March 20. (Source: National Phenology Network)

What is Phenology? It's the study of nature's calendar - when trees leaf and bloom, when birds begin nesting or leaves turn golden in a Colorado autumn.

According to the National Phenology Network, "Phenology refers to the science focused on understanding key seasonal changes in plants and animals from year to year, and how these seasonal events vary with weather and climate."

The USA National Phenology Network is a partnership among governmental and nongovernmental science and resource management agencies and organizations, the academic community and the public. It is led by, and receives major funding from, the US Geological Survey.

RELATED: Just HOW EARLY is spring arriving in your neighborhood? Find out . . .