"The difference between weather and climate is a measure of time. Weather is what conditions of the atmosphere are over a short period of time, and climate is how the atmosphere "behaves" over relatively long periods of time." [NASA]
Discussing weather events in the context of climate change isn’t simple. To be correct, we need to state that ‘on average’, the weather may be different. If an unusual weather event occurs, that one event can’t be attributed to climate change, but we can expect more such events over time.
One way in which humans can observe climate change on a personal level is taking notice of earlier flowering dates. The arrival of crocuses in the northern hemisphere is an eye-catching event. We notice it and see it as portending the end of winter, and may notice it happening earlier and earlier each year.
The study of the times of recurring natural phenomena is called ‘phenology‘.
For a tree to produce flower buds, open and flower, a succession of events must take place. It begins with a rise in the air temperature. This allows the vapour pressure at the leaf surfaces to reach a point for transpiration to occur. Sap begins to rise. Leaves require sunlight and a stable range of temperature; once leaves are set, a cold snap can kill them. Leaves on an tree can be taken, to anthropomorphise, as a sign of optimism and expectation that the long term weather will be favourable. The rising sap and water from the roots move sugars and starch around the tree. Sinks of sugars build up in the flower sites. When all these events have occurred in succession, flower development can begin. In the cherry tree, it is spectacular. Flowers grow, pollen forms, insects are drawn to the nectar and take the pollen with them to receptive flowers. These insects themselves need their own favourable chain of circumstance, and all these links in these chains are susceptible to the weather.
Long term, cold temperatures will delay the beginnings of the process. Brief, unusual warm periods in the winter can trigger the start of growth at too early a date.
Trees are an example of a very long term response. The amount of sunlight in the preceding year will affect the amount of fruit set in the next year. Plants respond in the short term to cold or hot snaps, of a day or a week, but when they flower, the average dates when crocuses emerge from the soil, when cherry trees bloom: these events are a response to the climate. If spring comes early, the chain begins early. Plants, in their lifecycles, integrate all the events that happen to them. Their fruit set or grain yield is the sum of the year of growth that preceded it. Plant growth is an averaging of weather, an integration of climate. When plants change their growth habits, take it as a sign that the climate has begun to change.